Materials design system and storage medium storing computer program for causing computer system to analyze stable structure or dynamic behavior of system at atomic or molecular level to assist materials design based on this analysis

ABSTRACT

Disclosed is a materials design system for obtaining the structural stability or dynamic behavior of an aperiodic system (S 2 ), which includes a reference system S 0  processing unit (1-1), an aperiodic system S 2  processing unit (1-2), an interatomic potential processing unit (1-3), a reference system S 0  calculation unit (1-4), an aperiodic system S 2  calculation unit (1-5), an MM calculation unit (1-6) for analyzing the stable structure, and an MD calculation unit (1-7) for analyzing the dynamic behavior. The structure of the aperiodic system (S 2 ) is described on the basis of a deviation from a reference system (S 0 ). When the potential energy of the aperiodic system (S 2 ) and the force acting on each atom or each molecule of the aperiodic system (S 2 ) are to be calculated, calculation is performed within a unit cell of the reference system (S 0 ) and a predetermined range (region where lattice relaxation is taken into consideration) containing the disorder in the aperiodic system (S 2 ). According to this arrangement, in materials design, the stable structure or dynamic behavior of a system containing a disorder (aperiodic system (S 2 )) can be efficiently calculated without approximation.

BACKGROUND OF THE INVENTION

The present invention relates to an assist system (CAD system) for designing a substance or a material using a computer and, more particularly, to MM (Molecular Mechanics) calculation and MD (Molecular Dynamics) calculation.

As is generally known, when a substance or a material is to be designed using a computer, the structural stability or dynamic behavior of a crystal system is analyzed using MM (Molecular Mechanics) calculation or MD (Molecular Dynamics) calculation.

In MD calculation, a change over time in motion of atoms or molecules at a finite temperature is tracked in accordance with a Newton's equation. This is one of essential techniques for substance/material design as a means for checking the microscopic behavior of a system (reference [1]: Akira Ueda, "Computer Simulation", Asakura Shoten (1990)).

MM calculation is a method of analyzing the most stable structure of a system and corresponds to MD calculation at the absolute zero (T=0 K) (reference [2]: Isao Okada and Eiji Osawa, "Introduction to Molecular Simulation", Kaibundo (1989)).

The MM calculation and MD calculation can be applied to both an infinite system (e.g., a crystal) and an isolated system (e.g., a molecule). The MM/MD calculation of the present invention is directed to the infinite system.

The "infinite system" means a system having a sufficient range at the atomic/molecular level, i.e., a solid or liquid such as a crystal, an amorphous substance, or a liquid crystal. Since such an infinite system cannot be directly handled by computer simulation, a cell (unit cell) having a predetermined size and called a unit cell is set in the infinite system. Accordingly, the infinite system is perceived as a system in which the unit cells are periodically and three-dimensionally arranged.

With this arrangement, computer simulation for the infinite system can be performed only by performing calculation while setting a periodic boundary condition for the unit cell.

This technique using the periodic boundary condition is supposed to be adequate for a crystal system having a periodicity. However, this method is also applied, as an approximation method, to a system whose periodicity disappears because of impurities.

The impurity is the major factor (disorder) for canceling the periodicity. The impurity here means a substitutional/interstitial impurity atom or a defect (the three types of disorders will be referred to as "impurity atoms" hereinafter). A set of one or more impurity atoms existing relatively close to each other in a crystal is called an "impurity". In addition to the impurity, a destruction phenomenon or the like can also be considered as a disorder. In this specification, an infinite system in which such disorders are distributed at a relatively low concentration (a system in which there are almost no interactions between disorders) is defined as an "aperiodic system".

When the disorder is an impurity, the "aperiodic system" can be regarded as a "system containing an isolated impurity". The "system containing an isolated impurity" means an infinite system having only one "impurity", or an infinite system having a plurality of impurities which are sufficiently separated from each other and do not interact each other.

In other words, in such a system, the content of the "impurity" is very low. An example is a defect in a (compound) semiconductor. This defect is known as a DX center which gives a deep impurity level near the center of the energy gap of the semiconductor and largely affects the device characteristics of a semiconductor laser or the like.

In analyzing the structural stability or dynamic behavior of such an "aperiodic system" on the basis of the above-described MM calculation or MD calculation, a method of performing simulation while giving prominence on one unit cell under the "periodic boundary condition" is generally known as a "supercell method".

In the supercell method, only one disorder is arranged in the unit cell, and a sufficiently large unit cell size is set, thereby approximately describing a state wherein disorders do not interact each other. More specifically, in the supercell method, to realize the state of the above-described aperiodic system (when the disorder is an "impurity", "a system containing an isolated impurity"), normally, only one disorder is set in the unit cell. However, this disorder is automatically arranged in peripheral unit cells (to be referred to as "image cells") because of the periodic boundary condition. To perform proper calculation while minimizing interaction with these disorders, the unit cell size must be as large as possible.

However, in the supercell method, the unit cell size must be determined in advance before the start of simulation (the size cannot be changed during simulation), and the size is often determined (to be relatively small) in consideration of the balance between the calculation accuracy and the calculation time.

When the supercell method is used, the aperiodic system can be handled within the same framework as that of the calculation method for a periodic system. For this reason, the supercell method is generally used as a convenient method for MD calculation or MM calculation.

To perform MD calculation or MM calculation using the supercell method, the force acting on each constituent atom of the system must be calculated in each step of repetitive calculation of MM calculation or at each time (one step of repetitive calculation) of MD calculation on the basis of the interatomic interaction. The potential energy of the system is often simultaneously calculated on the basis of the interatomic interaction. When the interatomic interaction is a long-range interaction, calculation of the force or energy slowly converges. Particularly, for a Coulomb interaction described as an r⁻¹ function, accurate calculation can hardly be executed when the sum is simply calculated for pairs of atoms, as is known. Therefore, a method called an Ewald method is generally used when calculation of the force or energy according to the Coulomb interaction is performed for the above-described infinite system (a system with the periodic boundary condition) (reference [1]).

This Ewald method can be applied to calculate the lattice sum (the sum for equivalent atoms belonging to different unit cells) of not only the Coulomb interaction but also an interaction given by an r^(-n) function (multipole interaction or van der Waals force). This is an excellent method because, by appropriately speeding up convergence of calculation of the lattice sum in a real space and an imaginary space, the force or energy of the r^(-n) long-range interaction is efficiently calculated.

However, MM/MD calculation for a periodic system using the conventional method or for an aperiodic system to which the supercell method is applied has the following problems.

Most of the MM/MD calculation execution time is spent for calculation of the force (and the energy). When the interatomic interaction is a long-range interaction, the number of pairs which must be taken into consideration becomes enormous. Particularly, for the Coulomb interaction, the calculation time still poses a serious problem even when the Ewald method is applied.

For MD calculation, when a temperature fluctuation is to be suppressed within the allowance in terms of statistical mechanics, the MD calculation must be performed for a system in which a number N of atoms per unit cell is at least several hundred. For this reason, it is an important challenge to minimize the amount of calculation of the force.

Especially, for the purpose of designing a substance/material, the MD calculation must be repeatedly performed for a number of systems while changing the simulation conditions including the temperature. Therefore, shortening of the calculation time, i.e., efficient calculation of the force is an important key to the practical new materials design.

In addition, since one step of the repetitive calculation in MD calculation corresponds to a time of 1 picosecond or less (about 0.01 PS) in a real system, the repetitive calculation must be performed in several ten thousand to several hundred thousand or more steps to analyze a change over time in system within a physically significant range. For this reason, for example, several ten days are still necessary to perform MD calculation once regardless of the improvement of the computer capability.

The above problems of the calculation time are posed when the long-range interaction is taken into consideration for both the periodic system and the aperiodic system handled by the supercell method. MM/MD calculation for the aperiodic system also has the following problems.

When the aperiodic system is to be handled by the supercell method, calculation must be performed using a unit cell with a sufficiently large size such that interaction between disorders contained in different unit cells can be neglected, as described above.

However, when the interatomic interaction is a long-range interaction, it is practically difficult to execute calculation using a sufficiently large unit cell because of the balance between the calculation time and the accuracy, resulting in a calculation error depending on the cell size. More specifically, in a normal simulation, calculation is often executed using a unit cell with a relatively small size in order to shorten the calculation time. In this case, the interaction with disorders contained in the peripheral unit cells cannot be neglected depending on the unit cell size, and a calculation error corresponding to the unit cell size is generated. In other words, the situation of the "aperiodic system" defined above cannot be exactly handled.

To confirm the dependency on the unit cell size, MM/MD calculation must be performed while changing the unit cell size, and the results must be compared. The Coulomb interaction as the most representative interaction is described by an r⁻¹ function. For this reason, to confirm convergence of the energy at, e.g., a 10-times higher accuracy, MM/MD calculation must be performed using a unit cell whose number N of atoms is 10³ times larger.

However, since the calculation time increases in proportion to the square of N, as described above, even confirmation of the dependency on the unit cell size can hardly be performed for the Coulomb interaction.

When the Coulomb interaction (charges) is considered in the aperiodic system, the following problem is generated in the supercell method. When the sum of charges of local disorders is not 0, the sum of charges per unit cell also deviates from 0 due to introduction of such disorders (e.g., an ion, a substitutional/interstitial impurity atom, and a defect). In the supercell method for which the periodic boundary condition is assumed, the sum of charges per unit cell must absolutely be 0 to prevent divergence of the energy, i.e., a restriction is technically required for the convenience of numerical calculation.

As a result of this restriction, when a system containing a disorder which breaks the charge neutrality is to be handled by the supercell method, and the sum of charges per unit cell is not 0, artificial charges (e.g., uniform charges) must be added to each constituent atom to nullify the sum. This disables calculation using the real charge distribution in the system. Particularly, consideration of the artificial charges largely affects the potential energy value.

The problems of the conventional method of analyzing the periodic system or aperiodic system using the supercell method will be summarized below.

(1) In MM/MD calculation, most of the MM/MD calculation execution time is spent for calculation of the force (and the energy). When the interatomic interaction is a long-range interaction, the number of pairs which must be taken into consideration becomes enormous. Particularly, for the Coulomb interaction, the calculation time still poses a serious problem even when the Ewald method is applied.

(2) When calculation for the aperiodic system is to be performed on the basis of the supercell method, calculation must be performed using a unit cell with a sufficiently large size such that interaction between disorders contained in different unit cells can be neglected.

However, when the interatomic interaction is a long-range interaction, it is practically difficult to execute calculation using a sufficiently large unit cell because of the calculation time, resulting in a calculation error depending on the cell size.

To confirm the dependency on the unit cell size, MM/MD calculation must be performed while changing the unit cell size, and the results must be compared. For a pair potential, the calculation time increases in proportion to the square of the number N of atoms. Therefore, for the r^(-n) long-range interaction, even confirmation of the dependency on the unit cell size can hardly be performed.

(3) When analysis using the supercell method is to be performed for a system where the charge neutrality has been lost, the charge distribution must be corrected to nullify the total charges per unit cell. In this case, the total potential energy, the equilibrium lattice constant, the equilibrium atomic coordinates, and the like of the system undesirable change, so the charge distribution of the system where the charge neutrality has been lost cannot be strictly handled.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of the above situation, and has as its object to provide a materials design system capable of efficiently analyzing the stable structure or dynamic behavior of an infinite system regardless of whether the system is a periodic system or an aperiodic system.

(1) First Aspect of Present Invention

Arrangement

According to the first aspect of the present invention, there is provided a materials design system which analyzes a structural stability or dynamic behavior of a system (aperiodic system S²) having a local disorder at an atomic or molecular level, thereby assisting materials design, comprising: a reference system S⁰ processing unit for processing physical quantities of a system (reference system S⁰) having no disorder; an aperiodic system S² processing unit for processing physical quantities of the aperiodic system S² and a list for making constituent atoms of the reference system S⁰ to correspond with those of the aperiodic system S² in one-to-one correspondence to describe a structure of the aperiodic system S² on the basis of a deviation from the reference system S⁰ ; a reference system S⁰ calculation unit for calculating an energy of the reference system S⁰ and a force acting on each atom or each molecule of the reference system S⁰ ; an aperiodic system S² calculation unit for calculating an energy of the aperiodic system S² and a force acting on each constituent atom of the aperiodic system S² using the energy of the reference system S⁰ and the force acting on each constituent atom of the reference system S⁰, which are calculated by the reference system S⁰ calculation unit; and an aperiodic system S² MM/MD calculation unit for executing calculation (MM calculation or MD calculation) of the structural stability or dynamic behavior of the aperiodic system S² using a calculation result from the aperiodic system S² calculation unit.

Function

With the above arrangement, a high-speed MM/MD calculation for the aperiodic system S² can be realized in the following manner.

In the first aspect, when MM/MD calculation for the aperiodic system S² is to be executed, the reference system S⁰ (a system whose constituent atoms correspond to those of the aperiodic system S² in one-to-one correspondence) is simultaneously handled.

The reference system S⁰ may be either a system having lattice vibration caused by the temperature effect at a finite temperature or a system without lattice vibration caused by the temperature effect at the absolute zero. In the former case, the reference system S⁰ may be divided into a reference system S⁰ (a system which does not consider lattice vibration) and a periodic system S¹ (a system which considers lattice vibration) and handled, as in the invention described in claims 2 and 5.

In the first aspect, the reference system S⁰ processing unit processes the structure and potential parameters of the reference system S⁰, and then, the aperiodic system S² processing unit processes the structure and potential parameters of the aperiodic system S² and makes the constituent atoms of the reference system S⁰ to correspond to those of the aperiodic system S² in one-to-one correspondence.

The reference system S⁰ calculation unit calculates the force and energy of the reference system S⁰. Since the reference system S⁰ has a periodicity, equation (18) (to be described later) can be applied to calculate the force acting on each constituent atom. In addition, the efficiency of calculation of the lattice sum of an r^(-n) long-range interaction can be increased by the Ewald method.

When the reference system S⁰ does not consider lattice vibration caused by the temperature effect, the amount of calculation of the force can be small because the number N_(T=0) of atoms per unit cell V⁰ is relatively small.

Since MM calculation performs an analysis at the absolute zero (T=0 K), the amount of calculation of the force of the reference system S⁰ is small.

That is, according to the first aspect, the force acting on each constituent atom of the reference system S⁰ whose calculation amount is small is used to calculate the force acting on each constituent atom of the aperiodic system S², thereby increasing the calculation efficiency.

More specifically, in the present invention, the structure of the aperiodic system S² is described as a deviation from the reference system S⁰. With this method, MM/MD calculation for an infinite system containing a completely isolated disorder is realized without using any artificial periodic boundary condition, unlike the conventional method (supercell method).

For this purpose, the present invention has the aperiodic system S² processing unit. The aperiodic system S² processing unit sets the region (described as R_(relax) in the embodiments) where lattice relaxation caused by the locally arranged disorder is taken into consideration, and processed the coordinates and potential parameters of each atom in this region and the correspondence list for making the atoms to correspond to those of the reference system S⁰ in one-to-one correspondence. When the force (energy) of the aperiodic system S² is to be calculated by the aperiodic system S² calculation unit, the calculation result of the force of the reference system S⁰, which is calculated by the reference system S⁰ calculation unit, us used to increase the calculation efficiency. MM/MD calculation is executed by the MM/MD calculation unit using the force which has been calculated in the above way. With this processing, MM/MD calculation for the aperiodic system S² is realized with a higher accuracy at a higher speed than the supercell method.

(2) Second Aspect of Present Invention

Arrangement

According to the second aspect of the present invention, the materials design system according to claim 1, further comprises, when an analysis (MD calculation) of the dynamic behavior of the aperiodic system S² is to be performed at a finite temperature (temperature T>0 K); means for describing the structure of the aperiodic system S² on the basis of a deviation from a system (periodic system S¹) which has no local disorder but considers vibration of atoms which is caused by a temperature effect, the means for describing the structure of the aperiodic system S² comprising a periodic system S¹ processing unit for processing a physical quantity of the periodic system S¹ and a list for making constituent atoms of a reference system S⁰ (the system in which lattice vibration is not taken into account) to correspond to those of the periodic system S¹ in one-to-one correspondence to describe a structure of the periodic system S¹ on the basis of a deviation from the reference system S⁰, a periodic system S¹ calculation unit for calculating an energy of the periodic system S¹ and a force acting on each constituent atom of the periodic system S¹ using the energy of the reference system S⁰ and the force acting on each constituent atom of the reference system S⁰, which are calculated by the reference system S⁰ calculation unit, and a periodic system S¹ MD calculation unit for executing periodic system MD calculation to analyze the dynamic behavior of the aperiodic system S² as a deviation from the periodic system S₁ ; and the aperiodic system S² MM/MD calculation unit executes calculation (MD calculation) of the structural stability or dynamic behavior of the aperiodic system S² using a calculation result from the aperiodic system S² calculation unit and a calculation result from the aperiodic system S¹ MD calculation unit.

Function

This aspect of the present invention provides, when the analysis (MD calculation) of the dynamic behavior of the aperiodic system S² is to be performed at a finite temperature (temperature T>0 K), the means capable of realizing efficient MD calculation for the aperiodic system S² even when the unit cell of the reference system S⁰ is relatively large because of lattice vibration caused by the temperature effect.

More specifically, in the invention according to claim 2, the reference system S⁰ having a relatively large size because of lattice vibration caused by the temperature effect is called a period system Si, and the reference system S⁰ is regarded as a system which does not consider lattice vibration caused by the temperature effect. The structure of the periodic system S¹ is calculated using the force acting on each constituent atom of the reference system S⁰, thereby increasing the calculation efficiency. In addition, an accurate and high-speed MD calculation for the aperiodic system S² is realized.

The method of increasing the calculation efficiency by handling two system, i.e., the reference system S⁰ and the periodic system S¹ will be described later in association with the fourth aspect of the present invention. Note that the calculation unit called an MD calculation unit in the invention according to claim 1 is divided into a periodic system MD calculation unit and an aperiodic system MD calculation unit in the invention according to claim 2.

The invention according to claim 2 means that, in MD calculation for the aperiodic system S², three systems, i.e., the reference system S⁰, the periodic system S¹, and the aperiodic system S² are handled.

(3) Third Aspect of Present Invention

Arrangement

According to the third aspect of the present invention, in the materials design system according to claim 1, the aperiodic system S² processing unit comprises means for changing, during simulation, a region where lattice relaxation of the aperiodic system S² is taken into consideration.

Function

According to this arrangement, even when the range of lattice relaxation (structure deformation) is larger than the region which is set before the start of MM/MD calculation, the structural stability or dynamic behavior of the aperiodic system S² can be properly and efficiently calculated.

(4) Fourth Aspect of Preset Invention

Arrangement

According to the fourth aspect of the present invention, there is provided a materials design system which analyzes a dynamic behavior of atoms or molecules in a system (periodic system S¹) which has no local disorder but vibration of atoms which is caused by a temperature effect, thereby assisting materials design, comprising: a reference system S⁰ processing unit for processing a physical quantity of a system (reference system S⁰) which does not consider vibration of atoms which is caused by the temperature effect; a periodic system S¹ processing unit for processing a physical quantity of the periodic system S¹ and a list for making constituent atoms of the reference system S⁰ to correspond with those of the periodic system S¹ in one-to-one correspondence to describe a structure of the periodic system S¹ on the basis of a deviation from the reference system S⁰ ; a reference system S⁰ calculation unit for calculating an energy of the reference system S⁰ and a force acting on each atom or each molecule of the reference system S⁰ ; a periodic system S¹ calculation unit for calculating an energy of the periodic system S¹ and a force acting on each constituent atom of the periodic system S¹ using the energy of the reference system S⁰ and the force acting on each constituent atom of the reference system S⁰, which are calculated by the reference system S⁰ calculation unit; and a periodic system S¹ MD calculation unit for analyzing dynamic behavior of the periodic system using a calculation result from the reference system S⁰ calculation unit.

Function

With the above arrangement, a high-speed MD calculation for the periodic system S¹ can be realized I the following manner.

According to the fourth aspect, when MD calculation for the periodic system S¹ is to be executed, the reference system S⁰ (a system whose constituent atoms correspond to those of the periodic system S¹ in one-to-one correspondence) is simultaneously handled.

The reference system S⁰ corresponds to the structure of the periodic system S¹ at the absolute zero (T=0 K), so that this system has no lattice vibration caused by he temperature effect. For this reason, for the number N_(T=0) of atoms per unit cell U⁰ of the reference system S⁰ generally, N_(T=0) <<N_(T>0) holds, as compared to the number N_(T>0) of atoms per unit cell U¹ of the periodic system S¹. In a system such as an amorphous structure which originally has a relatively large minimum unit cell size, a situation N_(T=0) N_(T>0) may be generated. For such a system, an accurate and high-speed MD calculation can be realized using the calculation method according to claim 1 in which the reference system S⁰ is regarded as a system which considers lattice vibration caused by the temperature effect.

In this aspect, the reference system S⁰ processing unit processes the structure and potential parameters of the reference system S⁰, and the periodic system S¹ processing unit processes the structure and potential parameters of the periodic system S¹ and makes the constituent atoms of the reference system S⁰ to correspond to those of the periodic system S¹ in one-to-one correspondence.

The reference system S⁰ calculation unit calculates the force and energy of the reference system S⁰. Since the reference system S⁰ has a periodicity, equation (69) (to be described later) can be applied to calculate the force acting on each constituent atom. In addition, the efficiency of calculation of the lattice sum of an r^(-n) long-range interaction can be increased by the Ewald method.

The amount of calculation of the force can be small because the number N_(T=0) of atoms per unit cell U⁰ is relatively small.

More specifically, when the force (and eneregy) of the periodic system S¹ is to be calculated by the periodic system S¹ calculation unit, the following processing is performed. When the force acting on a constituent atom (k0) (kth atom contained in a fundamental cell ζ=0) of the periodic system S¹ is to be calculated, a region R_(cut) centered on the atom (k0) is assumed. As an interaction with each atom (k'ζ') in the region R_(cut), a correct value in the periodic system S¹ is calculated. As an interaction with each atom (k'ζ') outside the region R_(cut), an interaction in a system which neglects lattice vibration caused by the temperature effect, i.e., the reference system S⁰ is approximately used.

Since the interatomic interaction is most largely influenced by atoms at a close position, it is adequate to approximately handle only an interaction with atoms at a remote position.

The region R_(cut) is set such that the number of atoms outside the region R_(cut) larger than that inside the region R_(cut). Since the calculation result of the reference system S⁰ whose calculation amount is small is used to calculate interactions with the atoms outside the region R_(cut), the efficiency of calculation of the force of the periodic system S¹ can be increased.

In the present invention, the force calculated by the periodic system S¹ calculation unit is used to execute MD calculation by the periodic system MD calculation unit, thereby realizing MD calculation for the periodic system S¹ at a higher speed than that of the conventional method.

(5) Fifth Aspect of Present Invention

Arrangement

According to the fifth aspect of the present invention, in the materials design system according to claim 1 or 5, the reference system S⁰ processing unit comprises means for setting a dummy atom or a dummy molecule in the reference system S⁰.

Function

According to this arrangement, even when lattice relaxation (structure deformation) occurs in an arbitrary size, the structural stability or dynamic behavior of the aperiodic system S² or the periodic system S¹ can be accurately calculated by setting the dummy atom or dummy molecule in the reference system S⁰.

(6) Sixth Aspect of Present Invention

According to the sixth aspect of the present invention, there is provided a storage medium which stores a computer program for causing a computer system to analyze a structural stability or dynamic behavior of a crystal system (aperiodic system S²) having a local disorder at an atomic or molecular level to assist materials design, comprising: a reference system S⁰ processing procedure code of processing a physical quantity of a system (reference system S⁰) having no disorder; an aperiodic system S² processing procedure code of processing a physical quantity of the aperiodic system S² and a list for making constituent atoms of the reference system S⁰ to correspond with those of the aperiodic system S² in one-to-one correspondence to describe a structure of the aperiodic system S² on the basis of a deviation from the reference system S⁰ ; a reference system S⁰ calculation procedure code of calculating an energy of the reference system S⁰ and a force acting on each atom or each molecule of the reference system S⁰ ; an aperiodic system S² calculation procedure code of calculating an energy of the aperiodic system S² and a force acting on each constituent atom of the aperiodic system S² using the energy of the reference system S⁰ and the force acting on each constituent atom of the reference system S⁰, which are calculated by the reference system S⁰ calculation procedure code; and an MM/MD calculation procedure code of executing calculation (MM calculation or MD calculation) of the structural stability or dynamic behavior of the aperiodic system S² using a calculation result from the aperiodic system S² calculation procedure code.

(7) Seventh Aspect of Present Invention

According to the seventh aspect of the present invention, the storage medium described according to sixth aspect further comprises, when an analysis (MD calculation) of the dynamic behavior of the aperiodic system S² is to be performed at a finite temperature (temperature T>0 K): a procedure code of describing the structure of the aperiodic system S² on the basis of a deviation from a system (periodic system S¹) which has no local disorder but considers vibration of atoms which is caused by a temperature effect, the procedure code of describing the structure of the aperiodic system S² comprising, a periodic system S¹ processing procedure code of processing a physical quantity of the periodic system S¹ and a list for making constituent atoms of a reference system S⁰ (the system in which lattice vibration is not taken into account) to correspond to those of the periodic system S¹ in one-to-one correspondence to describe a structure of the periodic system S¹ on the basis of a deviation from the reference system S⁰, a periodic system S¹ calculation procedure code of calculating an energy of the periodic system S¹ and a force acting on each constituent atom of the periodic system S¹ using the energy of the reference system S⁰ and the force acting on each constituent atom of the reference system S⁰, which are calculated by the reference system S⁰ calculation procedure code, and a periodic system MD calculation procedure code of executing periodic system MD calculation to analyze the dynamic behavior of the aperiodic system S² as a deviation from the periodic system S¹ ; and the aperiodic system S² MM/MD calculation procedure code executes calculation (MD calculation) of the structural stability or dynamic behavior of the aperiodic system S² using a calculation result from the aperiodic system S² calculation procedure code and a calculation result from the aperiodic system S¹ MD calculation procedure code.

The function of the seventh aspect corresponds to that of the second aspect of the present invention.

(8) Eight Aspect of Present Invention

According to the eighth aspect of the present invention, in the storage medium according to the sixth aspect, the aperiodic system S² processing procedure code comprises a procedure code of changing, during simulation, a region where lattice relaxation of the aperiodic system S² is taken into consideration.

The function of the eighth aspect corresponds to that of the third aspect of the present invention.

(9) Ninth Aspect of Present Invention

According to the ninth aspect of the present invention, there is provided a storage medium which stores a computer program for causing a computer system to analyze a dynamic behavior of atoms or molecules in a system (periodic system S¹) which has no local disorder but vibration of atoms which is caused by a temperature effect, thereby assisting materials design, comprising: a reference system S⁰ processing procedure code of processing a physical quantity of a system (reference system S⁰) which does not consider vibration of atoms which is caused by the temperature effect; a periodic system S¹ processing procedure code of processing a physical quantity of the periodic system S¹ and a list for making constituent atoms of the reference system S⁰ to correspond with those of the periodic system S¹ in one-to-one correspondence to describe a structure of the periodic system S¹ on the basis of a deviation from the reference system S⁰ ; a reference system S⁰ calculation procedure code of calculating an energy of the reference system S⁰ and a force acting on each atom or each molecule of the reference system S⁰ ; and a periodic system calculation procedure code of calculating an energy of the periodic system S¹ and a force acting on each constituent atom of the periodic system S¹ using the energy of the reference system S⁰ and the force acting on each constituent atom of the reference system S⁰, which are calculated by the reference system S⁰ calculation procedure code; and a periodic system S¹ MD calculation procedure code for analyzing dynamic behavior of the periodic system using a calculation result from the reference system S⁰ calculation procedure code.

The function of the ninth aspect corresponds to that of the fourth aspect of the present invention.

(10) 10th Aspect of Present Invention

According to the 10th aspect of the present invention, in a storage medium according to the sixth or ninth aspect, the reference system S⁰ processing procedure code comprises a procedure code of setting a dummy atom or a dummy molecule in the reference system S⁰.

The function of the 10th aspect corresponds to that of the fifth aspect of the present invention.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a block diagram showing the arrangement of a materials design system according to the first embodiment;

FIG. 2 is a view for explaining the relationship between a reference system S⁰ and an aperiodic system S² ;

FIG. 3 is a view showing the constituent atoms of the reference system S⁰ and the aperiodic system S² shown in FIG. 2 while assigning atomic numbers (hζ) and serial numbers (i) to the atoms;

FIG. 4 is a table showing an example of a correspondence list corresponding to the reference system S⁰ and the aperiodic system S² in FIG. 3;

FIG. 5 is a table showing potential parameters A of substitutional and interstitial impurity atoms and defects in the reference system S⁰ and the aperiodic system S² ;

FIG. 6 is a flow chart for executing MM calculation described in the first embodiment;

FIG. 7 is a flow chart for executing MD calculation described in the first embodiment;

FIG. 8 is a flow chart for executing MM calculation described in the second embodiment;

FIG. 9 is a flow chart for executing MD calculation described in the second embodiment;

FIG. 10 is a flow chart for executing MM calculation described in the third embodiment;

FIG. 11 is a flow chart for executing MD calculation described in the third embodiment;

FIG. 12 is a view showing an example of calculation of the stable structure (relaxed structure) by MM calculation of NaCl containing a substitutional impurity ion;

FIG. 13 is a view showing an example of calculation of the stable structure (relaxed structure) by MM calculation of NaCl containing two substitutional impurity ions;

FIG. 14 is a block diagram showing the arrangement of a materials design system for executing MD calculation for a periodic system S¹ according to the fifth embodiment;

FIG. 15 is a block diagram showing the arrangement of a materials design system for executing MD calculation for an aperiodic system S² according to the sixth embodiment;

FIG. 16 is a view for explaining the relationship between the reference system S⁰ and the periodic system S¹ ;

FIG. 17 is a view for explaining the relationship among the reference system S⁰, the periodic system S¹, and the aperiodic system S² ;

FIG. 18 is a table showing a correspondence list [S⁰ S¹ ] and a correspondence list [S¹ S² ];

FIG. 19 is a correspondence table showing a comparison, between a periodic system S¹ and the aperiodic system S², of the potential parameters A and the masses m of substitutional and interstitial impurity atoms and defects handled in the MD calculation for an aperiodic system S² according to the sixth embodiment;

FIG. 20 is a flow chart for executing the MD calculation for the periodic system S¹ according to the fifth embodiment;

FIG. 21 is a flow chart showing a modification of the MD calculation for the periodic system S¹ according to the fifth embodiment; and

FIG. 22 is a flow chart for executing the MD calculation for the periodic system S² according to the sixth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The first to seventh embodiments of the present invention will be described below with reference to the accompanying drawing.

First Embodiment

MM/MD Calculation for Aperiodic System S²

The first embodiment of the present invention will be described first with reference to the accompanying drawing.

This embodiment corresponds to the invention described in claim 1. In this embodiment, a case wherein a "disorder" is an "impurity" will be described. In this case, the aperiodic system S² is also called an "isolated impurity system".

FIG. 1 is a block diagram showing the arrangement of a materials design system according to the first embodiment of the present invention. This system arrangement is basically common to the second, third, and fourth embodiments to be described later.

The materials design system of this embodiment comprises a reference system S⁰ processing unit 1-1, an aperiodic system S² processing unit 1-2, an interatomic potential processing unit 1-3, a reference system S⁰ calculation unit 1-4, an aperiodic system S² calculation unit 1-5, an MM calculation unit 1-6, and an MD calculation unit 1-7. In this embodiment, the processing operations of the respective blocks shown in FIG. 1 will be described first, and then, the flow of processing of the materials design system will be described using flow charts.

The primary characteristic feature of the calculation method of this embodiment is that the atomic coordinates of a system (aperiodic system S²) containing an isolated impurity as a calculation target are represented by a deviation from a system (reference system S⁰) in which no impurity is arranged at all. This will be described with reference to FIG. 2. Referring to FIG. 2, atoms indicated by solid lines are atoms constituting the reference system S⁰, and boxes indicated by solid lines represent the unit cells of the reference system S⁰.

FIG. 2 two-dimensionally shows nine unit cells. The central unit cell will be called a fundamental cell (ζ=0). Assume that an atom placed almost at the center of the fundamental cell (ζ=0) is replaced with another element (indicated by a bullet). This is a substitutional impurity atom. Lattice relaxation (structure deformation) occurs at its periphery because of the presence of such an impurity. This lattice relaxation normally occurs only in a finite region centered on the impurity, and this region is indicated by a dashed line in FIG. 2. Since the equilibrium atomic coordinates of the atoms in this region change due to the interaction from the impurity, for example, the atoms are arranged at positions indicated by dashed lines (gray bullets). Therefore, the aperiodic system S² in this embodiment corresponds to the reference system S⁰ in which atomic coordinates in the region have changed.

Such a structure can be very effectively represented by a deviation between the crystal structure of the reference system S⁰ and that of the aperiodic system S² (the deviation is indicated by arrows in FIG. 2, and this deviation will also be referred to as a relaxation amount hereinafter). The reason for this is as follows. Since the reference system S⁰ contains no impurity at all, the unit cell can be set to be relatively small. Accordingly, the data amount (coordinates of atoms contained in the unit cell, potential parameters, and so on) required to describe the crystal structure of the reference system S⁰ can be small. In addition, the amount of calculation of the potential energy or force can also be decreased.

Next, the deviation from the reference system S⁰ will be described. Since this deviation is present only in a finite region centered on the impurity in the aperiodic system S², the data amount (vectors representing the deviations of the respective atoms) is not so large. Since the potential energy or force in the aperiodic system S² is calculated on the basis of this deviation, the amount of this calculation is not so large.

In the conventional supercell method, calculation must be executed using a unit cell nine or more times (27 times as a three-dimensional size) larger than that of the reference system S⁰ to calculate lattice relaxation as shown in FIG. 2. This results in disadvantages as described in the above-mentioned (BACKGROUND OF THE INVENTION).

In this embodiment, the two systems associated with each other, i.e., the reference system S⁰ and the aperiodic system S² are handled. The physical quantities (the atomic coordinates, the potential parameters, the potential energies of the systems, forces acting on the respective atoms, and the like) of the two systems must be clearly discriminated. The physical quantities of the reference system S⁰ will be represented with a suffix "0", and those of the aperiodic system S² will be represented with a suffix "2" hereinafter, thereby discriminating the physical quantities.

Processing of the reference system S⁰ processing unit 1-1 shown in FIG. 1 will be describe first. As described above, since the reference system S⁰ contains no impurity at all, the unit cell size can be relatively small. When a perfect crystal is assumed as the reference system S⁰, the minimum unit cell of the crystal can be regarded as the unit cell of the reference system S⁰. Not only the perfect crystal but also an amorphous structure may be assumed as the reference system S⁰ as far as the amorphous structure has a periodicity.

The reference system S⁰ processing unit 1-1 receives the lattice constants of the reference system S⁰, the number N⁰ of atoms per unit cell, atomic coordinates r⁰ (h0).sub.α of N⁰ atoms, and potential parameters A⁰ (h0) of N⁰ atoms, and holds these values.

All the unit cells are equivalent under the periodic boundary condition. Attention is paid to a unit cell ζ=0, and the atomic coordinates and potential parameters are designated for this fundamental cell (ζ=0). The coordinate value r⁰ (h0).sub.α represents the α-direction coordinate of the hth atom contained in the fundamental cell (ζ=0). Coordinate value r⁰ (hζ).sub.α of an atom contained in an arbitrary unit cell is calculated on the basis of the lattice constants (fundamental translation vector εa.sub.λα) and the relative coordinates (x.sub.λ (h)) by equations (1) to (3) below:

    r.sup.0 (hζ).sub.α =r.sup.0 (ζ).sub.α +r.sup.0 (h).sub.α                                           (1)

    r.sup.0 (ζ).sub.α =ζ.sub.1 a.sub.1α +ζ.sub.2 a.sub.2α +ζ.sub.3 a.sub.3α               (2)

    r.sup.0 (h).sub.α =x.sub.1 (h)a.sub.1α +x.sub.2 (h)a.sub.2α +x.sub.3 (h)a.sub.3α              (3)

where λ is the suffix for discriminating the three fundamental translation vectors. The reference system S⁰ processing unit 1-1 may receive the relative coordinates x.sub.λ (h) in place of the atomic coordinates r⁰ (h0).sub.α and set r⁰ (h0).sub.α in accordance with the above equations.

The potential parameter A⁰ (h0) is an interatomic potential parameter held by the interatomic potential processing unit 1-3. In the case of Coulomb interaction, A⁰ (h0) corresponds to a point change. For the atomic coordinates r⁰ (h0).sub.α and the potential parameters A⁰ (h0), equations (4) and (5) below hold for an arbitrary cell ζ because of the periodic boundary condition:

    r.sup.0 (hζ).sub.α =r.sup.0 (h0 ).sub.α +r.sup.0 (ζ).sub.α                                      (4)

    A.sup.0 (hζ)=A.sup.0 (h0)                             (5)

Handling for describing the crystal structure of the reference system S⁰ is the same as that of the conventional supercell method. Handling of the potential parameters A⁰ (h0) is also associated with the processing operation of the interatomic potential processing unit 1-3 shown in FIG. 1 and will be described in detail in association with the interatomic potential processing unit 1-3.

Processing of the aperiodic system S² processing unit 1-2 shown in FIG. 1 will be described next. In this embodiment, some atoms of the above-described reference system S⁰ are replaced with impurity atoms. Accordingly, the atoms of the reference system S⁰ and those of the aperiodic system S² must be handled in one-to-one correspondence. In the aperiodic system S², the periodicity of the reference system S⁰ disappears because of the existence of the local impurity (FIG. 2). For this reason, the notation using the atomic numbers (hζ) representing the periodicity is not appropriate anymore, and serial numbers (i) must be set for the atoms of the aperiodic system S².

The above processing can be performed using the correspondence list [S⁰ S² ]. An example of this correspondence list [S⁰ S² ] is shown in FIG. 4. This table shows the correspondence list [S⁰ S² ] for the system shown in FIG. 2. In FIG. 3, atomic numbers are assigned to the constituent atoms of the reference system S⁰ and the aperiodic system S² in FIG. 2. The atomic numbers in FIG. 3 correspond to those shown in the correspondence list [S⁰ S² ] (FIG. 4). FIG. 3 will be referred hereinafter.

In the correspondence list [S⁰ S² ] shown in FIG. 4, the atomic numbers (hζ) of the reference system S⁰ which contains four (N⁰ =4) constituent atoms in the unit cell are registered. As the constituent atoms of the aperiodic system S² corresponding to the reference system S⁰, 21 serial numbers (i) are registered. The 21 atoms correspond to atoms contained in a region indicated by a dashed line in FIG. 3. As is apparent from the correspondence list [S⁰ S² ] in FIG. 4, the constituent atoms of the aperiodic system S² are made to correspond to those of the reference system S⁰ in one-to-one correspondence. This correspondence can also be uniquely made in accordance with for example an equation i=h+ζN⁰. For the descriptive convenience, the impurity atoms in the aperiodic system S² will be also represented by impurity numbers (σ).

The flow of the processing operation of the aperiodic system S² processing unit 1-2 will be described below mainly in association with the initial setting stage of MM/MD calculations. The aperiodic system S² processing unit 1-2 receives the number N^(imp) of impurity atoms contained in the aperiodic system S², numbers which indicate the N^(imp) impurity atoms, potential parameters A^(imp) (σ) of the N^(imp) impurity atoms, and a region R_(relax) where lattice relaxation is taken into consideration. The numbers which indicate the N^(imp) impurity atoms mean the numbers of atoms in the reference system S⁰, which are replaced with impurity atoms. More specifically, a set {(h₁,ζ₁), (h₂,ζ₂), . . . (h_(Nimp),ζ_(Nimp))} is designated.

The example shown in FIG. 3 contains only one impurity atom (N^(imp) =1), and a set {(h=20, ζ=0)} is designated. An arbitrary number of impurity atoms can be handled as N^(imp). The impurity atom to be designated can be contained in the fundamental cell (ζ=0) of the reference system S⁰ or an image cell (ζ≠0). The set {(h₁,ζ₁), (h₂,ζ₂), . . . , (h_(Nimp),ζ_(Nimp))} for designating the impurity atoms is made to correspond to a set of impurity numbers, i.e., {(σ₁), (σ₂), . . . , (σ_(Nimp))}. In the correspondence list [S⁰ S² ] shown in FIG. 4, the number of impurity atoms is 1, and an impurity number (σ=1) is assigned to the atom (i=2) of the aperiodic system S².

The region R_(relax) where lattice relaxation is taken into consideration will be described next. In the aperiodic system S², lattice relaxation occurs around the impurity. Since the lattice relaxation becomes almost zero at a position far from the impurity atom, the lattice relaxation can be calculated only in a finite region centered on the impurity atoms. As the region where the lattice relaxation is taken into consideration, for example, a sphere having the radius R_(relax) and centered on the center of gravity of the impurity atoms can be assumed. This region need not always be spherical. In this embodiment, the region where lattice relaxation is taken into consideration is generally represented by R_(relax). This region R_(relax) may be arbitrarily designated by the user or set on the side of the materials design system of this embodiment. The aperiodic system S² processing unit 1-2 counts the number N² of atoms contained in the region R_(relax) (N² does not means the square of N. In the present invention, the square of N is described as N×N). All the N^(imp) impurity atoms should be contained in the N² atoms, as a matter of course. In the example shown in FIG. 3, N² =21. The aperiodic system S² processing unit 1-2 calculates coordinates r² (i).sub.α of the N² atoms and holds the coordinate values. The coordinates r² (i).sub.α are calculated by equation (6) below:

    r.sup.2 (i).sub.α =r.sup.0 (i).sub.α +Δr(i).sub.α(6)

As described above, the atoms (i) of the aperiodic system S² are made to correspond to the atoms (hζ) of the reference system S⁰ in one-to-one correspondence on the basis of the correspondence list [S⁰ S² ]. Therefore, for r⁰ (i).sub.α of equation (6), equation (7) below holds on the basis of the correspondence list [S⁰ S² ] and the periodicity of the reference system S⁰ :

    r.sup.0 (i).sub.α =r.sup.0 (hζ).sub.α =r.sup.0 (h0).sub.α +r.sup.0 (ζ).sub.α            (7)

The value is easily calculated from r⁰ (h0).sub.α which has already been held by the reference system S⁰ processing unit 1-1.

Next, the second term of equation (6), i.e., Δr(i).sub.α will be described. As described above, in this embodiment, the crystal structure of the aperiodic system S² is described on the basis of the deviation from the reference system S⁰. The deviation corresponds to Δr(i).sub.α which physically represents lattice relaxation caused by impurity. When the stable structure of the aperiodic system S² is to be calculated by MM calculation on the basis of the calculation method of this embodiment, this relaxation amount Δr(i).sub.α is calculated. The initial values of Δr(i).sub.α are set at the initial setting stage of MM/MD calculation. These initial values may be arbitrarily designated by the user or automatically set at 0 by the materials design system of this embodiment.

Handling of the potential parameter of each atom of the aperiodic system S² will be described next. As described above, the aperiodic system S² processing unit 1-2 receives the potential parameters A^(imp) (σ) of the N^(imp) impurity atoms (σ). Accordingly, the aperiodic system S² processing unit 1-2 sets potential parameters A² (i) of the N² atoms (i) contained in the region R_(relax) by using equations (8) and (9):

    A.sup.2 (i)=A.sup.0 (i)(=A.sup.0 (hζ)=A.sup.0 (h0) . . . i≠σ                                           (8)

    A.sup.2 (i)=A.sup.imp (σ) . . . i=σ            (9)

Like the potential parameters A⁰ (h0) of the reference system S⁰, handling of the potential parameters A² (i) is associated with the processing operation of the interatomic potential processing unit 1-3. The interatomic potential processing unit 1-3 will be described below.

The interatomic potential processing unit 1-3 shown in FIG. 1 holds a function representing an interatomic interaction. In this embodiment, the interatomic interaction acting between the atom (i) and an atom (j) is described by equation (10):

    φ(i,j)=A(i)A(j)φ(r|i,j)                   (10)

where A(i) corresponds to the potential parameters A⁰ (h0) and A² (i) described about the reference system S⁰ processing unit 1-1 and the aperiodic system S² processing unit 1-2. When φ(ij) represents a Coulomb interaction, A(i)=q_(i), and φ(r|ij)=1/{r(ij)}, where q_(i) is the point charge of the atom (i) and r(ij) is the distance between the atoms (i) and (j).

As the interatomic interaction described by the function (10), there are interatomic interactions such as van der Waals force and multipole interaction described by r^(-n) functions, in addition to the Coulomb interaction. Since these interactions are generally taken into consideration in MM/MD simulations, it can be said that equation (10) represents the general function of interatomic interactions. The materials design system of this embodiment can simultaneously handle a plurality of interatomic interactions such as Coulomb interaction+van der Waals force+short-range repulsive force. In this case, the interatomic interaction φ(ij) between the atom (i) and the atom (j) is described by equations (11) and (12): ##EQU1## Regardless of whether one or a plurality of interatomic interactions are to be taken into consideration, handling is performed in the same manner. Therefore, a suffix μ will be omitted for the above-described reference system S⁰ processing unit 1-1 and the aperiodic system S² processing unit 1-2 and in the following description. In this embodiment, a description associated only with an interatomic potential will be made. The calculation method of this embodiment can also handle a molecular crystal. In this case, an intermolecular potential is processed like the interatomic potential.

As described about the aperiodic system S² processing unit 1-2, the atoms of the reference system S⁰ are made to correspond to those of the aperiodic system S² in one-to-one correspondence on the basis of the correspondence list [S⁰ S² ], and the potential parameters are related to each other by equations (8) and (9). According to equations (8) and (9), when the potential parameters of the reference system S⁰ are compared to those of the aperiodic system S², the impurity atoms of the aperiodic system S² have potential parameter values different from those of the corresponding atoms of the reference system S⁰. However, the other atoms of the aperiodic system S² have the same potential parameter values as those of the corresponding atoms of the reference system S⁰.

The materials design system of this embodiment can handle three types of impurities, i.e., a substitutional impurity atom, an interstitial impurity atom, and a defect. These impurities are discriminated on the basis of the relationship in the potential parameters between the reference system S⁰ and the aperiodic system S², as shown in FIG. 5. Of the three types of impurities shown in FIG. 5, the interstitial impurity atom must be particularly carefully handled. An atom corresponding to the interstitial impurity atom is not originally present in the reference system S⁰. To make the one-to-one correspondence based on the correspondence list [S⁰ S² ], a dummy atom must be set in the reference system S⁰ in advance. Since the potential parameter A⁰ (h0) of the dummy atom is always 0, as shown in FIG. 5, the potential energy or the force acting on each atom of the reference system S⁰ does not change even when the dummy atom is set.

In the materials design system of this embodiment, to obtain the stable structure or dynamic behavior of the aperiodic system S² by MM calculation or MD calculation, the potential energy and the force acting on each atom of the reference system S⁰ and the aperiodic system S² are calculated. This method will be described below.

The processing operation of the reference system S⁰ calculation unit 1-4 shown in FIG. 1 will be described first. Since the reference system S⁰ has a periodicity, handling of the reference system S⁰ is the same as that of the conventional supercell method. The reference system S⁰ calculation unit 1-4 calculates the potential energy of the reference system S⁰ and the force acting on each atom of the reference system S⁰. In this embodiment, a position vector r⁰ (hζ,h'ζ').sub.α between two atoms is defined by equation (13) below:

    r.sup.0 (hζ,h'ζ'.sub.α =r.sup.0 (h'ζ').sub.α -r.sup.0 (hζ).sub.α                            (13)

The sign in the equations of the potential energy and force to be described below is based on the definition of equation (13). For the potential energy, equation (14) is calculated for the atom (h0) using the interatomic potential represented by equation (10): ##EQU2## For the force, f⁰ (h0).sub.α is calculated in accordance with equations (15) and (16): ##EQU3## In this embodiment, e⁰ (h0) and f⁰ (h0) are called a partial potential energy and a partial force, respectively.

The summation ##EQU4## in equation (15) means that the term in which h'=h and ζ'=ζ is eliminated from the sum. The summation ##EQU5## represents that the sum is calculated for terms as many as possible until the convergence of the sum attains a sufficient accuracy. Since the terms added by this sum correspond to the interatomic potential itself or the differential coefficient of the interatomic potential, the value of each term generally becomes smaller as the interatomic distance increases. Therefore, ##EQU6## means that the terms are added while calculating interactions for sufficiently far atoms where the interatomic interactions become negligible.

In calculation of the Coulomb interaction, the efficiency of calculation can be increased using the Ewald method (reference 1) for equations (14) and (15). A potential energy E⁰ _(cell) per unit cell of the reference system S⁰ is obtained using equation (14), as follows: ##EQU7## In the conventional supercell method, E⁰ _(cell) or E⁰ _(cell) /N⁰ (potential energy per atom) is calculated. A value obtained by adding E⁰ _(cell) of all unit cells contained in the reference system S⁰ corresponds to a total potential energy E⁰ _(whole). An a component F⁰ (h0).sub.α of the force acting on each atom (h0) in the fundamental cell (ζ=0) of the reference system S⁰ is calculated using equation (15) as follows:

    F.sup.0 (h0).sub.α =A.sup.0 (h0)f.sup.0 (h0).sub.α(18)

In the conventional supercell method, MM calculation and MD calculation are executed using the value F⁰ (h0).sub.α.

The processing operation of the aperiodic system S² calculation unit 1-5 shown in FIG. 1 will be described next. The primary characteristic feature of this embodiment is that the relaxation amount Δr(i).sub.α from the reference system S⁰ is calculated to obtain, e.g., the stable structure of the aperiodic system S². In this embodiment, to obtain the relaxation amount Δr(i).sub.α, ΔE and ΔF(i).sub.α of the aperiodic system S² are calculated. The two quantities are defined by equations (19) and (20):

    ΔE=E.sup.2.sub.whole -E.sup.0.sub.whole              (19)

    ΔF(i).sub.α =F.sup.2 (i).sub.α -F.sup.0 (i).sub.α(20)

These values are obtained by subtracting the physical quantities (second term on the right-hand side) of the reference system S⁰ from the physical quantities (first term on the right-hand side) of the aperiodic system S². More specifically, ΔE represents the change in potential energy by lattice relaxation caused by the impurity doped in the reference system S⁰. Both E⁰ _(whole) and E² _(whole) are the total potential energies and therefore represent divergence quantities. On the other hand, the difference ΔE has a finite value. This corresponds to the fact that the lattice relaxation caused by the local impurity atoms occurs only within a finite range.

F⁰ (i).sub.α and F² (i).sub.α are forces acting on the atoms of the reference system S⁰ and the aperiodic system S², respectively. ΔE and ΔF(i).sub.α are defined by the above equations and calculated as follows. ΔE and ΔF(i).sub.α are represented using two terms as follows:

    ΔE=ΔE.sup.(a) +ΔE.sup.(b)                (21)

    ΔF(i).sub.α =ΔF.sup.(a) (i).sub.α +ΔF.sup.(b) (i).sub.α                                           (22)

The energy is represented as follows: ##EQU8##

    δr(i,j)=|r.sup.2 (i,j)|-|r.sup.0 (i,j)(25) ##EQU9## The force is represented as follows: ##EQU10##

    Δr(i,j).sub.α =r.sup.2 (i,j).sub.α -r.sup.0 (i,j).sub.α                                         (30)

    =(r.sup.2 (j).sub.α -r.sup.2 (i).sub.α)-(r.sup.0 (j).sub.α -r.sup.0 (i).sub.α)                 (31) ##EQU11## For e.sup.0 (i) and f.sup.0 (i).sub.α of equations (23) and (28), equations (35) and (36) hold on the basis of the correspondence list [S.sup.0 S.sup.2 ] and the periodicity of the reference system S.sup.0 :

    e.sup.0 (i)=e.sup.0 (hζ)=e.sup.0 (h0)                 (35)

    f.sup.0 (i).sub.α =f.sup.0 (hζ).sub.α =f.sup.0 (h0).sub.α                                          (36)

These values represent physical quantities calculated by the reference system S⁰ calculation unit 1-4 on the basis of equations (14) and (15). In the above equations, ##EQU12## represents the sum for N^(imp) impurity atoms (σ), and ##EQU13## means the sum for the N² atoms contained in the region R_(relax).

The above-described series of equations mathematically represent Taylor expansion. The physical quantities (E² whole and F² (i).sub.α) of the aperiodic system S² are Taylor-expanded on the basis of the deviation (δr(ij) or Δr(ij).sub.α) from the reference system S⁰. The first term in the equation (21) or (22) is given by subtracting the physical quantity (E⁰ whole or F⁰ (i).sub.α) from the zero-order term in the Taylor expansion. The second term in equation (21) or (22) corresponds to the higher-order term in the Taylor expansion. In equations (24) and (29) which give this higher-order term, terms of second or lower order are presented, and O[(δr)³ ] and O[(Δr)³ ] represent terms of third or higher arbitrary order.

Instead of equations (24) and (29), in equations (106) and (108) to be described later, equations in which the quantities of a periodic system S¹ are replaced with those of the reference system S⁰ may be used.

The processing operation of the MM calculation unit 1-6 shown in FIG. 1 will be described next. The MM calculation unit 1-6 calculates the stable structure of the aperiodic system S² by MM calculation on the basis of the calculation method of this embodiment. In the MM calculation, the potential energy of the system is minimized to obtain the stable structure. Several calculation methods are available as a means for realizing this calculation. In this embodiment, a method of steepest descent will be exemplified. According to the materials design system of this embodiment, when MM calculation is to be executed using repetitive calculation based on the method of steepest descent, in the nth step, a deviation εΔr(i|n).sub.α of the N² atoms contained in the region R_(relax) of the aperiodic system S² is calculated according to equation (37):

    Δr(i|n).sub.α =Δr(i|n-1).sub.α +εΔF(i|n-1).sub.α            (37)

where ΔF(i).sub.α is calculated according to equation (22) by the aperiodic system S² calculation unit 1-5. In addition, a constant ε may be arbitrarily designated by the user or set on the side of the materials design system of this embodiment to efficiently execute the MM calculation. When Δr(i).sub.α is calculated by the MM calculation unit 1-6 in accordance with the above equation, the aperiodic system S² processing unit 1-2 updates the value Δr(i).sub.α, and simultaneously, calculates the atomic coordinates r² (i).sub.α of the aperiodic system S². The above processing is performed using repetitive calculation until ΔF(i|n).sub.α becomes almost zero under a certain determination condition. With this processing, the stable structure of the aperiodic system S² can be obtained.

According to the calculation method of this embodiment, the stable structure of the aperiodic system S² can be obtained by equation (37), and simultaneously, the normal MM calculation can be performed for the atomic coordinates r⁰ (h0).sub.α of the reference system S⁰. However, to increase the calculation efficiency, the stable structure (equilibrium structure) of the reference system S⁰ is preferably input in advance, or before MM calculation for the aperiodic system S² is started, MM calculation for the reference system S⁰ is preferably independently performed to obtain the stable structure of the reference system S⁰. During this MM calculation, the aperiodic system S² calculation unit 1-5 can calculate ΔE on the basis of equation (21) in an arbitrary step. The difference ΔE physically represents the change in potential energy which is caused by the lattice relaxation by the impurity doped in the reference system S⁰, as described above. When ΔE is calculated in each step, and the resultant value is monitored, the process of minimizing the potential energy of the aperiodic system S² by the MM calculation can be tracked.

The processing operation of the MD calculation unit 1-7 shown in FIG. 1 will be described next. In the MD calculation, the velocity of each atom is also calculated. Therefore, when MD calculation is to be executed, the reference system S⁰ processing unit 1-1 and the aperiodic system S² processing unit 1-2 additionally hold v⁰ (h0).sub.α and v² (i).sub.α, respectively, representing the velocity of each atom. Masses m⁰ (h0) and m² (i) are also held. Accordingly, a supplementary explanation about the reference system S⁰ processing unit 1-1 and the aperiodic system S² processing unit 1-2 will be made, and then, the processing operation of the MD calculation unit 1-7 will be described. For the values v⁰ (h0).sub.α and m⁰ (h0) held by the reference system S⁰ processing unit 1-1, equations (38) and (39) hold on the basis of the correspondence list [S⁰ S² ] and the periodicity of the reference system S⁰ :

    v.sup.0 (i).sub.α =v.sup.0 (hζ)(=v.sup.0 (h0).sub.α(38)

    m.sup.0 (i)=m.sup.0 (hζ)=m.sup.0 (h0)                 (39)

These values are designated by the user at the initial setting stage of MD calculation. Alternatively, these values may be set on the side of the materials design system of this embodiment in accordance with a certain reference. Handling of the mass of each atom is the same as that of the potential parameter and can also be treated as one of potential parameters. Next, the aperiodic system S² processing unit 1-2 performs, for v² (i).sub.α, the same processing as that for r² (i).sub.α. More specifically, a description based on equation (20) below, which is equal to equation (6), is made:

    v.sup.2 (i).sub.α =v.sup.0 (i).sub.α +Δv(i).sub.α(40)

The initial value of Δv(i).sub.α is set. This initial value may be arbitrarily designated by the user, or set at 0 on the side of the materials design system of this embodiment at once. Since the mass m² (i) of each atom can be regarded as one of potential parameters, as described above, the same processing as for A² (i) is performed. More specifically, a mass m_(imp) (σ) of the N^(imp) impurity atom is input to the aperiodic system S² processing unit 1-2, and accordingly, the masses m² (i) of the N² atoms (i) are set in accordance with equations (8) and (9). The supplementary explanation about the reference system S⁰ processing unit 1-1 and the aperiodic system S² processing unit 1-2 has been made above.

The processing operation of the MD calculation unit 1-7 will be described. In this embodiment, a Verlet method will be exemplified as the algorithm of MD calculation. For the reference system S⁰, the position and velocity of each of N⁰ atoms (h0) contained in the fundamental cell (ζ=0) at a time t are calculated on the basis of equations (41) and (42): ##EQU14## The reference system S⁰ processing unit 1-1 updates these values. F⁰ (h0).sub.α is the α-direction force of the atom (h0), which is calculated by the reference system S⁰ calculation unit 1-4 in accordance with equation (18). A value Δt represents the time interval in MD calculation. The value Δt may be arbitrarily designated by the user, or set on the side of the materials design system of this embodiment. MD calculation for the reference system S⁰ according to equations (41) and (42) is handled as in the conventional supercell method.

For the aperiodic system S², Δr(i).sub.α and Δv(i).sub.α of the N² atoms (i) contained in the region R_(relax) at a time t are calculated in accordance with equations (43) and (44): ##EQU15## Accordingly, the aperiodic system S² processing unit 1-2 updates these values, and simultaneously, calculates r² (i).sub.α and v² (i).sub.α in accordance with equation (6) or (40). A value ΔF'(i).sub.α is calculated by the aperiodic system S² calculation unit 1-5 in accordance with equations (45) to (47) obtained by slightly changing equations (28) and (29):

    ΔF'(i).sub.α =ΔF.sup.(a) '(i).sub.α +ΔF.sup.(b) '(i).sub.α                        (45) ##EQU16## When m.sup.0 (i)=1, and m.sup.2 (i)=1 in equations (46) and (47), these equations are equivalent to equations (28) and (29). For this reason, for the aperiodic system S.sup.2 calculation unit 1-5, equation (45) may be prepared in place of equation (22), and MM calculation may be executed assuming that m.sup.0 (i)=1, and m.sup.2 (i)=1.

In MD calculations generally, the total energy (kinetic energy+potential energy) of the system is monitored to confirm the energy conservation law. In the calculation method of this embodiment as well, the potential energy can be calculated at the arbitrary time t. For the aperiodic system S², E⁰ _(cell) is calculated, and for the reference system S⁰, ΔE is calculated. The MD calculation unit 1-7 executes repetitive calculation based on equations (41) to (44) until a time t_(end). The time t_(end) can be arbitrarily designated by the user.

The processing operation in the block diagram (FIG. 1) showing the arrangement of the materials design system of this embodiment has been described in units of blocks. Next, to clarify the connection state among the blocks, MM calculation will be described below with reference to the flow chart shown in FIG. 6. First, physical quantities of the reference system S⁰ are input to the reference system S⁰ processing unit 1-1 (step 2-1). The physical quantities are the lattice constants, the number N⁰ of atoms per unit cell, the coordinates r⁰ (h0).sub.α of the N⁰ atoms, and the potential parameters A⁰ (h0) of the N⁰ atoms.

Next, physical quantities of the aperiodic system S² are input to the aperiodic system S² processing unit 1-2 (step 2-2). The physical quantities are the number N^(imp) of impurity atoms, the numbers of the N^(imp) impurity atoms (σ), the potential parameters A^(imp) (σ) of the impurity atoms (σ), and the region R_(relax) where lattice relaxation is taken into consideration.

When these physical quantities are input, the aperiodic system S² processing unit 1-2 prepares the correspondence list [S⁰ S² ]. The coordinates r² (i).sub.α and the potential parameters A² (i) of the N² atoms contained in the region R_(relax) in the aperiodic system S² are set, and the initial values of Δr(i).sub.α are set (step 2-3).

The reference system S⁰ calculation unit 1-4 calculates the partial potential energies e⁰ (h0) and the partial forces f⁰ (h0).sub.α of the reference system S⁰ (step 2-4).

The above steps correspond to initial setting. Subsequent steps correspond to the processing operation of repetitive calculation in MM calculation. First, ΔE and ΔF(i).sub.α are calculated by the aperiodic system S² calculation unit 1-5 (step 2-5). The relaxation amount Δr(i).sub.α is calculated by the MM calculation unit 1-6 using the value ΔF(i).sub.α (step 2-6). Accordingly, the aperiodic system S² processing unit 1-2 updates the value Δr(i).sub.α and calculates the atomic coordinates r² (i).sub.α of the aperiodic system S² (step 2-7).

The repetitive calculation from step 2-5 to step 2-7 is executed until the value ΔF(i).sub.α becomes sufficiently small under a certain determination condition. When the determination condition is satisfied, r² (i).sub.α is output or displayed as the stable structure (relaxed structure) of the aperiodic system S² (step 2-8). In steps 2-4 and 2-5, the potential energy e⁰ (h0) or ΔE need not always be calculated, and the calculation can be arbitrarily executed in accordance with the purpose of the user.

MD calculation will be described next with reference to the flow chart shown in FIG. 7. From step 3-1 to step 3-3 in FIG. 7, the same processing as described in FIG. 6 in association with MM calculation is executed. However, in this processing associated with MD calculation, the velocities v⁰ (h0).sub.α and the masses m⁰ (h0) of the atoms in the reference system S⁰ are additionally input in step 3-1. In step 3-2, the masses m^(imp) (σ) of the impurity atoms are also input. In step 3-3, setting of the values v² (i).sub.α and m² (i) is performed in addition to the processing in step 2-3.

In the MD calculation, processing from step 3-4 corresponds to repetitive calculation processing. First, e⁰ (h0) and f⁰ (h0).sub.α are calculated by the reference system S⁰ calculation unit 1-4 (step 3-4), and ΔE and ΔF(i).sub.α are calculated by the aperiodic system S² calculation unit 1-5 (step 3-5). The MD calculation unit 1-7 calculates r⁰ (h0).sub.α and v⁰ (h0).sub.α for the reference system S⁰ and Δr(i).sub.α and Δv(i).sub.α for the aperiodic system S² (step 3-6). The reference system S⁰ processing unit 1-1 and the aperiodic system S² processing unit 1-2 respectively update the values r⁰ (h0).sub.α and Δr(i).sub.α and r² (i).sub.α is calculated by the aperiodic system S² processing unit 1-2 (step 3-7).

Processing from step 3-4 to step 3-7 is repeatedly executed until the time tend, and the structure of the aperiodic system S² at the time tend is output or displayed (step 3-8). In the MD calculation of this embodiment, r⁰ (h0).sub.α and v⁰ (h0).sub.α for the reference system S⁰, or r² (i).sub.α and v² (i).sub.α for the aperiodic system S² can be output or displayed at an arbitrary time during the repetitive calculation. In steps 3-4 and 3-5, e⁰ (h0) and ΔE need not always be calculated every time and can be calculated at an arbitrary time in accordance with the purpose of the user.

Processing as the framework of the materials design system according to this embodiment has been described above.

Modifications of the materials design system of this embodiment will be described below as the second, third, and fourth embodiments.

Second Embodiment

The second embodiment corresponds to the invention described in claim 3.

First, handling of a region R_(relax) where lattice relaxation is taken into consideration, which is set by an aperiodic system S² processing unit 1-2 will be described.

In the materials design system of the first embodiment, the region R_(relax) is set such that the impurity is positioned almost at the center of the region R_(relax). For this reason, the relaxation amount Δr(i).sub.α is generally large at the central portion of the region R_(relax) and becomes almost zero at the edge of the region R_(relax). In MM calculation and MD calculation, the relaxation amount Δr(i).sub.α is updated by the aperiodic system S² processing unit 1-2 in each step or at each time point of the repetitive calculation. When the relaxation amount Δr(i).sub.α becomes relatively large even at the edge of the region R_(relax) during a simulation, this means that the range of real lattice relaxation is larger than the region R_(relax), and no proper calculation result can be obtained by this simulation.

To prevent this, the calculation method of the second embodiment allows to change the size of the region R_(relax) during the simulation. The region R_(relax) is changed in the following manner. For MM calculation, the flow chart in FIG. 6 changes to that in FIG. 8, so that step 2-9 and 2-10 are added. In FIG. 8, the relaxation amount Δr(i).sub.α is updated in every step of the repetitive calculation in step 2-7, as has already been described. In step 2-9, the value of the relaxation amount Δr(i).sub.α at the edge of the region R_(relax) is determined in accordance with a certain reference. If it is determined that the relaxation amount Δr(i).sub.α at the edge of the region R_(relax) is larger than a reference value Δr^(max) _(edge), the flow advances to step 2-10. In step 2-10, the aperiodic system S² processing unit 1-2 resets the region R_(relax).

This resetting can be executed by, e.g., replacing the radius R_(relax) of the region with a radius R'_(relax) =R_(relax) +R_(add). The newly set region R'_(relax) contains N'² (>N²) atoms. In step 2-10, information associated with the newly counted (N'² -N²) atoms is additionally registered in the correspondence list [S⁰ S² ], and simultaneously, the initial values of Δr(i).sub.α of these atoms are set. After the above processing is performed in step 2-10, the flow returns to the normal repetitive calculation. The above-described reference value Δr^(max) _(edge) and R_(add) may be arbitrarily designated by the user or set on the side of the materials design system of this embodiment in advance to increase the calculation efficiency.

When the region R_(relax) is appropriately changed following the above procedures, a proper calculation result (relaxed structure) can always be obtained. For MD calculation, the flow chart in FIG. 7 changes to that in FIG. 9. In steps 3-9 and 3-10 added in FIG. 9, the same processing as that in steps 2-9 and 2-10 is performed.

Third Embodiment

The third embodiment corresponds to the invention described in claims 4 and 6.

Processing performed when a relaxation amount Δr(i)_(a) becomes large even at the central portion of a region R_(relax) during a simulation will be described next as the third embodiment.

The materials design system of the first embodiment is characterized in that the atomic coordinates of the aperiodic system S² are described on the basis of the deviation from the reference system S⁰, and the physical quantities (the potential energy and the force acting on each atom) of the aperiodic system S² are approximately calculated by Taylor expansion associated with Δr(i).sub.α.

Generally, in Taylor expansion, when the expansion variable is small (near the expansion point), the accuracy of approximation is guaranteed. However, when the expansion variable becomes large (a point separated from the expansion point), the calculation accuracy degrades, as is known. In the materials design system of the first embodiment, the reference system S⁰ corresponds to the expansion point. When the deviation from the expansion point, i.e., the relaxation amount Δr(i).sub.α becomes large, the calculation accuracy of the physical quantities (ΔE and ΔF(i).sub.α) which are calculated using Taylor expansion gradually degrades.

To prevent this, in the third embodiment, accurate calculation can always be performed in accordance with the following procedures. For MM calculation, the flow chart in FIG. 6 changes to that in FIG. 10. In step 2-11 of FIG. 10, it is determined whether the value Δr(i).sub.α for each of the N² atoms contained in the region R_(relax) is larger than a reference value Δr^(max) _(whole). If YES in step 2-11, the flow advances to step 2-12. The reference value Δr^(max) _(whole) may be arbitrarily designated by the user or set on the side of the materials design system in advance. A case wherein it is determined that the relaxation amount Δr(i).sub.α of each of N_(big) (<N²) atoms is larger than the reference value Δr^(max) _(whole) will be described below. The N_(big) atoms are represented by (i_(big)). The atoms of the reference system S⁰, which are made to correspond to those of the aperiodic system S² in accordance with the correspondence list [S⁰ S² ], are represented by (h_(big),ζ_(big)).

The following processing is performed when step 2-12 is executed for the first time on the basis of the determination result in step 2-11. In step 2-12, addition/changing processing for the reference system S⁰ is performed by the reference system S⁰ processing unit 1-1. First, N_(big) dummy atoms (h_(dmy) ⁰) (to be described as (h_(dmy) ; ζ=0) hereinafter) are additionally set in the fundamental cell of the reference system s⁰. With this processing, the number of atoms per unit cell of the reference system S⁰ becomes N⁰ +N_(big). The potential parameter and atomic position of the newly set dummy atom (h_(dmy) ; ζ=0) of the reference system S⁰ are set in accordance with equations (48) and (49):

    A.sup.0 (h.sub.dmy ;ζ=0)=0                            (48)

    r.sup.0 (h.sub.dmy ;ζ=0).sub.α =r.sup.2 (i.sub.big).sub.α( 49)

In step 2-13, the aperiodic system S² processing unit 1-2 performs addition/changing processing for the aperiodic system S². Since, in step 2-12, the dummy atom is arranged in the reference system S⁰, the number of atoms contained in the aperiodic system S² also increases. First, atoms contained in the region R_(relax) of the aperiodic system S² are counted again, and a correspondence list of the dummy atom (h_(dmy) ;ζ) of the reference system S⁰ and the corresponding dummy atom (i_(dmy)) of the aperiodic system S² is added to the A correspondence list [S⁰ S² ]. When the number of atoms added to the correspondence list [S⁰ S₂ ] is represented by N_(add), N_(add) >N_(big). The specific value N_(add) is determined in consideration of the unit cell size of the reference system S⁰ and the size of the region R_(relax).

From step 2-13, the number of atoms contained in the region R_(relax) of the aperiodic system S² is represented by N² +N_(add). From step 2-13, in addition to the original impurity atom (σ), the atom (i_(big)) and the dummy atom (i_(dmy)) are also handled as impurity atoms. Subsequently, the potential parameters of the atom (i_(big)) and the dummy atom (i_(dmy)) are rewritten or newly set. For the atom (i_(big)), the potential parameter is changed as follows:

    A.sup.2 (i.sub.big)=0                                      (50)

Next, the potential parameter of the dummy atom is newly set. For setting, the following points must be taken into consideration.

The dummy atom (h_(dmy) ; ζ=0) arranged in the fundamental cell of the reference system S⁰ in step 2-12 is also arranged in the image cells (ζ≠0) because of the periodicity of the reference system S⁰. For this reason, two types of dummy atoms are added to the correspondence list [S⁰ S² ] in step 2-13. One is a dummy atom (i.sup.(ζ=0),_(dmy)) which is made to correspond to the dummy atom (h_(dmy) ; ζ=0) contained in the fundamental cell (ζ=0) of the reference system S⁰ in the correspondence list [S⁰ S₂ ]. The other is a dummy atom (i.sup.(ζ≠0),_(dmy)) which is made to correspond to a dummy atom (h_(dmy) ; ζ≠0) contained in the image cell (ζ≠0) of the reference system S⁰ in the correspondence list [S⁰ S₂ ]. The potential parameters of the two types of dummy atoms of the aperiodic system S² are set in accordance with equations (51) and (52) below: ##EQU17## The meanings of the two equations are as follows. After step 2-13, the physical quantities of the atom (i_(big)) originally present in the aperiodic system S² are calculated as the physical quantities of the dummy atom (i.sup.(ζ=0),_(dmy)). For this purpose, equation (51) is set. For the dummy atom (i.sup.(ζ≠0),_(dmy)) which is set because of the periodicity of the reference system S⁰ although it is unnecessary, the potential parameter is set to be 0 by equation (52), thereby setting a situation as if the dummy atom (i.sup.(ζ≠0),_(dmy)) were physically absent. For the relaxation amounts of these atoms, the initial value of Δr(i_(dmy)).sub.α is set to be 0. For Δr(i_(big)).sub.α, the value need not be changed because the relaxation amount becomes unnecessary from step 2-13.

The above series of processing operations corresponds to processing of shifting the expansion point of Taylor expansion to make the expansion variable small. More specifically, since the relaxation amount Δr(i_(big)).sub.α of the atom (i_(big)) becomes large, the dummy atom (h_(dmy) ; ζ=0) is virtually arranged at the position (r⁰ (h_(dmy) ; ζ=0).sub.α =r⁰ (i_(big)).sub.α +Δr(i_(big)).sub.α) of the reference system S⁰. Since the potential parameter A⁰ (h_(dmy) ; ζ=0) of this dummy atom is set to be 0 by equation (48), the potential energy or force of the reference system S⁰ does not change due to addition of the dummy atom (h_(dmy) ; ζ=0). After the above processing is performed for the reference system S⁰, processing for expressing the atomic coordinates of the atom (i_(big)) by r² (i.sup.(ζ=0),_(dmy)).sub.α instead of r² (i_(big)).sub.α is performed for the aperiodic system S². More specifically, the coordinates of the atom (i_(big)) are represented using r² (i.sup.(ζ=0),_(dmy)).sub.α as follows: ##EQU18## In MM calculation from step 2-12, as the relaxation amount of the atom (i_(big)), Δr(i.sup.(ζ=0),_(dmy)).sub.α is obtained instead of Δr(i_(big)).sub.α.

When the dummy atom (i_(dmy)) is arranged, the number of atoms contained in the aperiodic system S² apparently increases. However, since the original potential parameter A² (i_(big)) of the atom (i_(big)) is changed to 0 by equation (50), and the potential parameter of the dummy atom (i.sup.(ζ≠0),_(dmy)) is also set to be 0 by equation (52), the physical number of atoms contained in the aperiodic system S² after step 2-13 is equal to that before step 2-13.

It appears that the number of atoms (number of pairs) which must be taken into consideration in calculation of the potential energy or force abruptly increases due to setting of the dummy atoms. However, since the potential parameters A of most dummy atoms are 0, terms associated with the dummy atoms need not be handled in sum calculation by the reference system S⁰ calculation unit 1-4 or the aperiodic system S² calculation unit 1-5. Therefore, the increase in calculated amount of the potential energy or force caused by setting of the dummy atoms is not so large.

In step 2-12, the crystal structure of the reference system S⁰ is apparently changed by setting the dummy atoms. Accordingly, after step 2-13, the flow temporarily returns to step 2-4 to execute calculation of e⁰ (h0) and f⁰ (h0).sub.α again, and then, repetitive calculation based on the normal MM calculation is continuously executed.

When processing from step 2-11 to step 2-13 is performed, MM calculation can be executed with a sufficient calculation accuracy even for the aperiodic system S² having a very large lattice relaxation. Therefore, the versatility of the materials design system of this embodiment largely increases.

The following points must be taken into consideration. The above-mentioned processing in steps 2-12 and 2-13 is performed in the first cycle of the processing. After the second or subsequent cycles of steps 2-12 and 2-13, the processing changes as follows. If it is determined in step 2-11 twice or more that the relaxation amount Δr(i).sub.α of the same atom is larger than the reference value Δr^(max) _(whole), the dummy atom corresponding to this atom has already been set in the reference system S⁰. Therefore, in steps 2-12 and 2-13 from the second cycle, setting of the dummy atom and setting/change of the potential parameter need not be performed, and only equation (49) is calculated. More specifically, only the expansion point r⁰ (h_(dmy) ; ζ0).sub.α need be reset, and the initial value of Δr(i_(dmy)).sub.α need be set to be 0 again.

The same processing can also be performed for MD calculation. In this case, the flow chart in FIG. 7 is changed to that in FIG. 11. Processing from step 3-11 to step 3-13 is the same as that from step 2-11 to step 2-13.

The flow charts shown in FIGS. 10 and 11 are procedures for realizing highly accurate MM/MD calculation for the aperiodic system S² in which lattice relaxation with an arbitrary size occurs. To obtain the same function as that of these procedures, dummy atoms may be prepared in advance at the initial setting stage. More specifically, dummy atoms corresponding to all of the N² atoms (i) contained in the region R_(relax) of the aperiodic system S² may be set in the reference system S⁰ and the aperiodic system S² in advance. With this arrangement, processing of newly setting the dummy atoms in steps 2-12 and 2-13 in FIG. 10 (or steps 3-12 and 3-13 in FIG. 11) can be omitted.

Modifications of the first embodiment associated with MM calculation and MD calculation have been described above as the second and third embodiments. The basic flow chart associated with MM calculation is shown in FIG. 6, and FIGS. 8 and 10 are flow charts of the modifications. For the descriptive convenience, the two modifications have been independently described. However, a materials design system having the both functions can also be formed. For the MD calculation as well, a materials design system having both the functions shown in FIGS. 9 and 11 which show modifications of the basic flow chart in FIG. 7 can be formed.

Fourth Embodiment

The fourth embodiment will be described below.

In this embodiment, handling of interatomic interactions described by functions other than the function given by equation (10) will be described. Processing to be described below can be applied to all the first to third embodiments described above. As the short-range repulsive force between atoms, a Born-Mayer type function represented by equation (55) is often used: ##EQU19## where B(i) is the potential parameter corresponding to the effective radius of an atom (i). The function including the potential parameter B(i) is represented by equation (56):

    φ(i,j)=A(i)A(j)φ(B,r|i,j)                 (56)

A function ψ of equation (10) includes no potential parameters. However, in equation (56), a function ψ includes a potential parameter B. When an interatomic potential having the function given by equation (56) is to be handled by the materials design system of this embodiment, the following processing is performed.

As in the description of the potential parameter A, the potential parameter B of a reference system S⁰ is represented by B⁰ (h0), and the potential parameter B of an aperiodic system S² is represented by B² (i). B² (i) is represented by equation (57):

    B.sup.2 (i)=B.sup.0 (i)+ΔB(i)                        (57)

For B⁰ (i), equation (58) holds because of the correspondence list and the periodicity of the reference system S⁰ :

    B.sup.0 (i)=B.sup.0 (hζ)=B.sup.0 (h0)                 (58)

When B² (i) is represented using equation (57), only an impurity atom (σ) of the aperiodic system S² has a finite value ΔB(σ) (≠0), and the values ΔB(i) of all the remaining atoms are 0.

An aperiodic system S² calculation unit 1-5 calculates physical quantities ΔE and ΔF(i).sub.α of the aperiodic system S² as follows. These two quantities are represented by three terms instead of equations (21) and (22):

    ΔE=ΔE.sup.(a) +ΔE.sup.(b) +ΔE.sup.(c)(59)

    ΔF(i).sub.α =ΔF.sup.(a) (i).sub.α +ΔF.sup.(b) (i).sub.α +ΔF.sup.(c) (i).sub.α         (60)

For the energy, the respective terms are represented by equations (61) to (64): ##EQU20## For the force, the terms can be represented as follows: ##EQU21## Equations (61), (62), (65), and (66) have the same meanings as those of equations (23), (24), (28), and (29). In equations (61) and (65), the potential parameter B⁰ of the reference system S⁰ is used for calculation. In equations (62) and (66), the potential parameter B² of the aperiodic system S² is used for calculation.

An important point in handling the interatomic interaction represented by the function given by equation (56) is that the third terms of equations (59) and (60) correspond to terms of higher order in Taylor expansion of ΔB. In equations (63) and (67) which give this term of higher order, terms of first order are rewritten, and O[(ΔB)² ] represents terms of second or higher arbitrary order.

When the physical quantities ΔE and ΔF(i).sub.α of the aperiodic system S² are calculated in the above manner, MM/MD calculation using the interatomic potential represented by the general function given by equation (56) can be performed by the calculation method of this embodiment, so that interatomic interactions including the Born-Mayer type interaction can be handled.

For an interatomic interaction represented by a function which is not described by equation (10), the potential energy and force may be calculated by the conventional method, and the resultant potential energy and force may be added to the potential energy and force calculated by the method of the present invention to execute MM/MD calculation.

According to the materials design systems of the above-described first to fourth embodiments, the following effects can be obtained, unlike the conventional supercell method.

(1) Shortening of the computational time

(2) Reduction of the calculation load

(3) Realization of calculation for a perfect isolated system

(4) Realization of calculation without correction for a system where the charge neutrality has been lost

These four effects will be described below in more detail on the basis of the actual results of MM calculation.

Shortening of the computational time will be described first. For the descriptive convenience, NaCl as a typical ionic crystal will be exemplified as a simple crystal system. The NaCl has an fcc structure in which Na atoms having positive charges and Cl atoms having negative charges are alternated. Assume the reference system S⁰ as the perfect crystal of NaCl and the aperiodic system S² in which only one Na ion in the reference system S⁰ is substituted with a Cl ion. FIG. 12 shows results obtained by calculating the stable structure (relaxed structure) of the aperiodic system S² by MM calculation based on the calculation method of this embodiment and the conventional supercell method. ⋄ indicates the calculation result obtained by the calculation method of this embodiment, and □ indicates the calculation result obtained by the supercell method.

In this calculation example, the unit cell for the supercell method is set to have 512 atoms (solid line). To compare the calculation accuracies between the method of the present invention and the conventional method, for the calculation method of the present invention, a sphere having almost the same volume as that of the unit cell for the supercell method is set as the region R_(relax) where lattice relaxation is taken into consideration (dashed line). The unit cell of the reference system S⁰ for the calculation method of the present invention is assumed to have eight atoms, i.e., the minimum unit cell as a cubic crystal. In FIG. 12, an atom (solid atom) positioned at the center is a substitutional impurity atom upon substituting Na with Cl. As is apparent from FIG. 12, lattice relaxation occurs around the substitutional impurity atom. The calculation result obtained by the calculation method of the present invention is the same as that obtained by the conventional supercell method.

Attention need be paid to the fact that comparison of the calculation results must be made only for the fundamental cell of the supercell method. The reason for this is as follows. In the supercell method, the image cells also have impurity atoms because of the periodicity. For this reason, in FIG. 12 which shows the calculation result obtained by the supercell method, a total of nine impurity atoms and lattice relaxation around each impurity atom are shown. However, according to the calculation method of this embodiment, a completely isolated impurity system can be handled, so that a proper calculation result can be obtained throughout the whole region of the crystal. In the example shown in FIG. 12, however, since the region of lattice relaxation is not so large, the unit cell containing 512 atoms used for the supercell method has a sufficient size, and for the fundamental cell, the same calculation result as that of the calculation method of this embodiment can be obtained.

The fact that the two calculation results are equal for the fundamental cell means that although approximation calculation based on Taylor expansion is performed in the calculation method of the present invention, a sufficient calculation accuracy can be attained. A more important point is that the calculation method of this embodiment can efficiently shorten the calculation time to about 80 times that of the conventional supercell method. The reason why the calculation time can be largely shortened will be described below.

The calculation time of MM/MD calculation is mainly determined by the number of pairs of atoms which must be taken into consideration in calculation of the potential energy or force. Calculation of the force will be described below. In the supercell method, the force acting on each atom is calculated in accordance with equation (18). In equation (18), f⁰ (h0).sub.α is calculated using equation (15). In this calculation, the differential of the interatomic potential must be calculated for all pairs of atoms contained in the unit cell and atoms interacting with the atoms in the crystal. The number of pairs largely increases when the unit cell size increases. In the supercell method, when the unit cell size is increased to realize a "system containing an isolated impurity", the calculation time largely increases.

To prevent this, according to the present invention, calculation of equation (15) which requires a long calculation time is executed only for a small unit cell (e.g., the minimum unit cell of the crystal), thereby allowing to short the calculation time. This is specifically realized by calculation of ΔF(i).sub.α based on equation (22). As has been already described, this calculation is given by equations (28) and (29).

First, f⁰ (h0).sub.α is calculated in accordance with equation (28). This is the physical quantity of the reference system S⁰. Since the unit cell of the reference system S⁰ is small, the calculation amount is not so large.

Next, ##EQU22## included in equation (28) is the sum only for the impurity atoms (σ). Since the number of impurity atoms is generally several to several ten, the calculation amount is very small. ##EQU23## included in equation (29) means that calculation is performed for all pairs of atoms contained in the region R_(relax) and atoms interacting with the atoms. In this case, the fact that the term to be calculated in this sum includes Δr(ij).sub.α must be taken into consideration. This Δr(ij).sub.α abruptly approaches 0 as it is separated from the impurity. For this reason, the number of pairs which must be taken into consideration in ##EQU24## is not so large, and convergence of calculation is very good.

Due to the above-described reason, the number of pairs which must be taken into consideration in the calculation method of this embodiment is much smaller than that of the conventional supercell method, so that the calculation time can be shortened. In calculation of f⁰ (h0).sub.α of equation (28), the Ewald method (reference 1) can be used for the Coulomb interaction. Alternatively, the efficient method used in the prior art can be directly applied. In addition, f⁰ (i).sub.α is a physical quantity of the reference system S⁰. For this reason, when an equilibrium structure is to be used as the reference system S⁰ in MM calculation, calculation need be performed once at the initial setting stage, and thereafter, can be omitted. Therefore, the calculation time can be further shortened.

The above description about calculation of the force also applies to calculation of the potential energy ΔE. Due to the above-described reason, according to the calculation method of the present invention, the calculation time can be made much shorter than that of the conventional supercell method.

The second effect of the present invention, i.e., reduction of the calculation load will be described next. NaCl will be exemplified again, and this time, a system in which two Na ions are substituted with Cl ions is assumed. FIG. 13 shows results obtained by simulating the stable structure of this system by MM calculation based on the calculation method of the present invention and the conventional supercell method (two solid atoms indicate impurity atoms).

In FIG. 13, the two calculation results do not match even for the fundamental cell of the supercell method and, more particularly, at the edge of the fundamental cell, unlike the above-described FIG. 12. In image cells adjacent to the fundamental cell, the shift between the two calculation results becomes more conspicuous. This means that, in the example shown in FIG. 13, since the range of lattice relaxation is large, the unit cell (containing 512 atoms) assumed for the supercell method has no sufficient size, and the calculation result obtained by the supercell method cannot give a proper stable structure (relaxed structure). To confirm whether the calculation result obtained by the supercell method is correct, calculation must be performed a number of times while changing the unit cell size. An appropriate unit cell size is predicted and set, and calculation is performed for confirmation. Accordingly, not only the excess calculation load but also an excess calculation time is required for the supercell method.

On the other hand, in the calculation method of the present invention, the region R_(relax) where lattice relaxation is taken into consideration can be appropriately changed during the simulation, as described above in the flow charts shown in FIGS. 8 and 9. Even when the initially set region R_(relax) is smaller than the actual lattice relaxation range, a proper calculation result (relaxed structure in case of MM calculation) can be obtained by performing only one simulation, so that the load on the user and the calculation time can be reduced. Actually, in the calculation example based on the calculation method of the present invention shown in FIG. 13, the region R_(relax) where lattice relaxation is taken into consideration is larger than that of the example shown in FIG. 12.

The third effect of the calculation method of this embodiment will be described next. In the conventional supercell method, the impurity arranged in the fundamental cell is inevitably present in the peripheral image cells because of the periodic boundary condition, so the state of an "isolated impurity" cannot be properly realized unless the unit cell size is not sufficiently large. Normally, the unit cell size can hardly be increased due to the balance to the calculation time, and calculation must often be performed for an approximate system (a system in which interaction between impurities cannot be completely neglected). An example of unsatisfactory approximation corresponds to the calculation result of the supercell method shown in FIG. 13.

On the other hand, according to the calculation method of the present invention, the aperiodic system S² has no periodicity, so the interaction with the "impurity" contained in the image cells is not originally present in the aperiodic system S², unlike the supercell method. For this reason, simulation for a "completely isolated impurity" can always be performed.

Finally, the fourth effect of the present invention will be described. In the supercell method, when the sum of charges in the unit cell is shifted from 0 by setting impurity atoms, the charges of the impurity atoms themselves peripheral atoms must be corrected. In such a case, no real charge distribution can be handled. In the calculation example by the supercell method shown in FIG. 13, two Na ions are substituted with Cl ions, and the sum of charges per unit cell is -4. To correct the charges, in the calculation shown in FIG. 13, a charge of +4/512 is uniformly added to all atoms in the unit cell in the supercell method. When the charge distribution is corrected in this way, the potential energy of the system, the equilibrium lattice constant, or the equilibrium atomic coordinates undesirably change.

However, the aperiodic system S² need not satisfy this charge neutrality condition. Therefore, calculation can be executed even for a system where the charge neatrality has been lost as shown in FIG. 12 or 13, without artificially correcting charges.

Fifth Embodiment

MD Calculation for Periodic System

The fifth embodiment of the present invention will be described below with reference to the accompanying drawing.

The fifth embodiment corresponds to the invention described in claim 5.

Prior to a description of this embodiment, calculation of a force for an infinite system with a periodic boundary condition will be briefly described.

An interatomic interaction acting between an atom (hζ) and an atom (h'ζ') is represented by φ(hζ,h'ζ') where (hζ) represents the hth atom contained in the ζth unit cell, an a component of a force acting on an atom (h0) contained in a unit cell (ζ=0) is represented by equation (69): ##EQU25## where r(hζ,h'ζ').sub.α (=r(hζ).sub.α -r(h'ζ').sub.α) is the a component of a positional vector between the two atoms, ##EQU26## is the differential coefficient obtained by differentiating the interatomic potential by atomic coordinates, N is the number of atoms per unit cell, ##EQU27## is the sum for N atoms contained in a unit cell, and ##EQU28## is the sum for equivalent atoms which have the same value h' contained in different unit cells. Calculation of the latter ##EQU29## corresponds to the above-described lattice sum, and "all" means that sum calculation is performed for pairs as many as possible until the convergence attains a sufficient accuracy.

The lattice sum ##EQU30## needs the longest computational time in the calculation of equation (69), and the above-described Ewald method is essentially applied to an r^(-n) long-range interaction such as a Coulomb interaction. Most execution time required for MD calculation is spent to calculate the force according to equation (69), and the calculation time can be estimated as follows.

In a system containing N atoms per unit cell, the three components of the force acting on each of the N atoms are calculated using the above equation, so the calculation amount is represented by 3×N×N×N_(all). The second N corresponds to calculation of ##EQU31## and N_(all) corresponds to calculation of ##EQU32##

The Ewald method is generally applied to calculation of the lattice sum, as described above. The calculation amount can hardly be represented as a specific numerical value, and therefore, represented by N_(all) in this case. The value N_(all) is always larger than the number N of atoms per unit cell (N_(all) >>N). In MD calculation, repetitive calculation must be performed generally several thousand to several ten thousand times to track the change over time in the system. Therefore, the total calculation amount for the force is N×N×N_(all) ×N_(step) (coefficient "3" is omitted).

This calculation amount increases in proportion to the square of the number N of atoms per unit cell. For this reason, when the cell size increases, the calculation time becomes much longer. The cell size generally used for the MD calculation (supercell method) corresponds to several hundred to several thousand atoms. Recently, calculations for a system containing several million or more atoms are also performed by using a super parallel computer. The number N of atoms per unit cell is determined depending on the system. For a periodic system and an aperiodic system, the lower limit values of N are as follows.

Periodic system . . . N_(T>0) which gives a temperature fluctuation with a statistically allowable accuracy.

Aperiodic system . . . N_(SC) which gives a sufficiently large cell size capable of neglecting the interaction between disorders artificially arranged in the unit cells due to the periodic boundary condition.

The periodic system will be described first. In MD calculation, the motion of atoms or molecules is analyzed at a finite temperature (T>0 K). For this reason, each atom microscopically vibrates due to the temperature effect. Inversely speaking, since this vibration gives the temperature of the system, the temperature of the system can be calculated at each time point (a step of repetitive calculation) of the MD calculation using the vibration (velocity) of each atom. The temperature fluctuation (time dependence) is determined depending on (in proportion to 1/√N) of the number of independent atoms in the system (when the periodic boundary condition is set, the number N of atoms per unit cell).

To realize a designated temperature T with a small number N of atoms per unit cell, each atom must excessively largely vibrate. Consequently, the temperature fluctuation increases in proportion to 1,√N, and this MD calculation handles a physically abnormal system.

For this reason, for the periodic system, the MD calculation must be executed in a system having a cell size larger than the number N_(T>0), with which a small temperature fluctuation can be given with a statistically allowable accuracy.

When the aperiodic system is to be approximately handled by the supercell method, a virtual disorder is arranged in each unit cell due to the periodic boundary condition. To represent the real aperiodic system (system without interaction between disorders), a sufficiently large unit cell size must be set. N_(SC) for giving the sufficiently large unit cell has a larger value in a system having a long-range interaction.

The fifth embodiment will be described below on the basis of the above-described procedures of MD calculation.

FIG. 14 is a block diagram showing the arrangement of a materials design system for executing MD calculation for a periodic system S¹. The materials design system comprises a reference system S⁰ processing unit 4-1, a periodic system S¹ processing unit 4-2, an interatomic potential processing unit 4-4, a reference system S⁰ calculation unit 4-5, a periodic system S¹ calculation unit 4-6, and a periodic system MD calculation unit 4-8. In the fifth embodiment, the processing operations of the respective blocks in FIG. 14 will be described first, and then, the flow of processing of executing MD calculation for the periodic system S¹ according to the present invention will be described with reference to flow charts.

Processing performed by the reference system S⁰ processing unit 4-1 shown in FIG. 14 will be described first. The reference system S⁰ processing unit 4-1 holds/processes the crystal structure, the potential parameters, and the like of the reference system S⁰. The reference system S⁰ corresponds to the structure of the periodic system S¹ at the absolute zero (T=0 K) and has a periodicity. The unit (unit cell U⁰) of the periodicity of the reference system S⁰ is given by, e.g., the minimum unit cell of a crystal. As will be described later, the unit cell U⁰ is the constituent unit of a unit cell U¹ of the periodic system S¹.

The reference system S⁰ processing unit 4-1 receives the lattice constants of the reference system S⁰, the number N⁰ of atoms per unit cell U⁰, atomic coordinates r⁰ (h0).sub.α of N⁰ atoms, potential parameters A⁰ (h0), and masses m⁰ (h0) and holds these values.

The number N⁰ of atoms per unit cell U⁰ corresponds to the above-described N_(T=0). A suffix (hζ) represents the hth atom in the ζth unit cell of the reference system S⁰. All the unit cells are equivalent under the periodic boundary condition. Therefore, prominence is given to the 0th (ζ=0) unit cell, and the atomic coordinates, the potential parameters, and the like are designated for this fundamental cell (ζ=0).

The atomic coordinate r⁰ (h0).sub.α represents the a component of the coordinate of the hth atom contained in the fundamental cell (ζ=0). Coordinate r⁰ (hζ).sub.α of an atom contained in an arbitrary unit cell is calculated from the lattice constant (fundamental translation vectors α.sub.λα) and relative coordinates x.sub.λ (h) of the atom as follows:

    r.sup.0 (hζ).sub.α =r.sup.0 (ζ).sub.α +r.sup.0 (h).sub.α                                           (70)

    r.sup.0 (ζ).sub.α =ζ.sub.1 a.sub.1α +ζ.sub.2 a.sub.2α +ζ.sub.3 a.sub.3α ζλ:(71)

    r.sup.0 (h).sub.α =x.sub.1 (h)a.sub.1α +x.sub.2 (h)a.sub.2α +x.sub.3 (h)a.sub.3α              (72)

    0≦x.sub.λ (h)≦1

where λ is the suffix for discriminating the three fundamental translation vectors.

The reference system S⁰ processing unit 4-1 may receive the fundamental translation vectors a.sub.λα and the relative coordinates x.sub.λ (h) instead of atomic coordinates r⁰ (h0).sub.α and set the atomic coordinates r⁰ (h0).sub.α in accordance with the above equations. For the atomic coordinates r⁰ (h0).sub.α, the potential parameters A⁰ (h0), and the masses m⁰ (h0), equations (73) to (75) hold for an arbitrary cell ζ caused by the periodic boundary condition:

    r.sup.0 (hζ).sub.α =r.sup.0 (h0).sub.α +r.sup.0 (ζ).sub.α                                      (73)

    A.sup.0 (hζ)=A.sup.0 (h0)                             (74)

    m.sup.0 (hζ)=m.sup.0 (h0)                             (75)

Handling of the potential parameter A⁰ is associated with the processing operation of the interatomic potential processing unit 4-4 shown in FIG. 14 and will be described later in detail in association with the interatomic potential processing unit 4-4.

The periodic system S¹ processing unit 4-2 shown in FIG. 14 will be described next. The periodic system S¹ processing unit 4-2 holds/processes the structure, the potential parameters, and the like of the periodic system S¹. The periodic system S¹ is a system in which microscopic vibration of each atom in the reference system S⁰, which is caused by the temperature effect, is taken into consideration. The unit cell U¹ of the periodic system S¹ is constituted by M unit cells U⁰ of the reference system S⁰.

As described above, when the number of atoms (corresponding to N_(T>0)) contained in the unit cell U¹ is small, the computational time necessary for MD calculation can be short. However, since the temperature fluctuation of the system increases, MD calculation with a statistically required accuracy cannot be realized. For this reason, in MD calculation, generally, a system whose number of atoms per unit cell is several hundred or more is used for calculation.

The periodic system S¹ processing unit 4-2 receives the lattice constants of the periodic system S¹, the number N¹ of atoms per unit cell U¹, atomic coordinates r¹ (k0).sub.α of N¹ atoms, velocities v¹ (k0).sub.α, potential parameters A¹ (k0), and masses m¹ (k0) and holds these values. The number N¹ of atoms per unit cell U¹ corresponds to the above-described N_(T>0).

Instead of directly designating N¹, a temperature fluctuation ΔT with an accuracy required by the user may be designated, and N¹ may be determined from the temperature fluctuation ΔT.

A suffix (kξ) represents the kth atom in the ξth unit cell of the periodic system S¹. All the unit cells are equivalent under the periodic boundary condition. Therefore, prominence is given to the 0th (ξ=0) unit cell, and the atomic coordinates, the potential parameters, and the like are designated for this fundamental cell (ξ=0).

As described above, the unit cell U¹ of the periodic system S¹ is constituted by M(=M₁ ×M₂ ×M₃) unit cells U⁰ of the reference system S⁰. Atoms in the periodic system S¹ are made to correspond to those of the reference system S⁰ in one-to-one correspondence. The correspondence can be made for an atom (k0) in the fundamental cell (ξ=0) of the periodic system S¹ and an atom (hζ) in the reference system S⁰ using a relation k=h+ζN⁰ (k=1, . . . , N¹). This information is stored in a correspondence list [S⁰ S¹ ].

FIG. 16 shows an example of the correspondence between the reference system S⁰ (constituent atoms and unit cells are indicated by dashed lines) and the periodic system S¹ (constituent atoms and unit cells are indicated by solid lines). FIG. 16 shows a two-dimensional structure in which the unit cell U¹ of the periodic system S¹ is constituted by nine (M=9) unit cells U⁰ of the reference system S⁰.

FIG. 18 shows the correspondence list [S⁰ S¹ ] for FIG. 16. In the example shown in FIG. 16, the number N⁰ of atoms per unit cell U⁰ of the reference system S⁰ is 4, and the number N¹ of atoms per unit cell U¹ of the periodic system S¹ is 36. In the periodic system S¹ processing unit 4-2, instead of directly inputting the values of the lattice constants of the periodic system S¹, the number N¹ of atoms per unit cell U¹, the atomic coordinates r¹ (k0), of N¹ atoms, the potential parameters A¹ (k0), and the masses m¹ (k0) by the user, information (M₁ ×M₂ ×M₃) may be input as follows:

The number N¹ of atoms per unit cell U¹ is calculated as N⁰ ×M on the basis of this information, thereby preparing the correspondence list [S⁰ S¹ ]. At this time, in the above relation k=h+ζN⁰, assume that ζ=(ζ₁,ζ₂,ζ₃), (ζ₁ =1, . . . , M₁, ζ₂ =1, . . . , M₂, ζ₃ =1, . . . , M₃). In addition, on the basis of the correspondence list [S⁰ S¹ ] and the periodicity of the reference system S⁰, the initial values of the atomic coordinates r₁ (h0).sub.α, the values of the potential parameters A¹ (h0) and the masses m¹ (h0) may be set as follows:

    r.sup.1 (k0).sub.α =r.sup.0 (hζ).sub.α =r.sup.0 (h0).sub.α +r.sup.0 (ζ).sub.α            (76)

    A.sup.1 (k0)=A.sup.0 (hζ)=A.sup.0 (h0)                (77)

    m.sup.1 (k0)=m.sup.0 (hζ)=m.sup.0 (h0)                (78)

For the velocities v¹ (k0).sub.α, when MD calculation is to be performed at a temperature T, initial velocities corresponding to this temperature must be set. Initial velocities for giving the temperature T, which are designated by the user, may be automatically generated by the materials design system of the present invention or appropriately set by the user.

The interatomic potential processing unit 4-4 shown in FIG. 14 holds a function representing an interatomic interaction. In this embodiment, the interatomic interaction acting between the atom (i) and the atom (j) is described by a function (79) below:

    φ(i,j)=A(i)A(j)φ(r|i,j)                   (79)

where A(i) corresponds to potential parameters A⁰ (hξ), A¹ (kξ), and A² (i) described in the description of the reference system S⁰ processing unit 4-1, the periodic system S¹ processing unit 4-2, and the aperiodic system S² processing unit 4-3. When φ(i,j) represents a Coulomb interaction,

A(i)=qi (qi is the charge of the atom (i)), and

ψ(r|i,j)=1/r(i,j) (r(i,j) is the interatomic distance between the atom (i) and the atom (j)).

In addition to the Coulomb interaction, a van der Waals force, a multipole interaction, and the like are interatomic interactions represented by a function such as equation (79). These interactions are generally taken into consideration in MD calculations, and equation (79) is the representative function of the interatomic interactions. The materials design system of the present invention can simultaneously handle a plurality of interatomic interactions such as Coulomb interaction+van der Waals force+short-range repulsive force. In this case, the interatomic interaction φ(i,j) acting between the atom (i) and the atom (j) is described by equations (80) and (81): ##EQU33## Regardless of whether only one or a plurality of interatomic interactions are to be taken into consideration, handling is performed in the same manner. Therefore, a suffix μ for discriminating the interatomic interactions will be omitted for the above-described reference system S⁰ processing unit 4-1 and the periodic system S¹ processing unit 4-2 and the following description. In this embodiment, a description associated only with an interatomic potential will be made. The materials design system of the present invention can also handle an intermolecular potential.

In the materials design system of the present invention, to perform MD calculation for the periodic system S¹, the energies of the reference system S⁰ and the periodic system S¹ and a force acting on each atom are calculated. This method will be described below.

The processing operation of the reference system S⁰ calculation unit 4-5 shown in FIG. 14 will be described. The reference system S⁰ calculation unit 4-5 calculates the potential energy of the reference system S⁰ and the force acting on each atom. First, the α component of the positional vector r⁰ (hζ,h'ζ').sub.α between two atoms is defined as follows:

    r.sup.0 (hζ,h'ζ').sub.α =r.sup.0 (hζ).sub.α -r.sup.0 (h'λ').sub.α                        (82)

The position vector between two atoms of the periodic system S¹ or aperiodic system S² (to be described later) is also defined by equation (82). The reference system S⁰ calculation unit 4-5 calculates physical quantities represented by e⁰ (h0) and f⁰ (h0).sub.α using equations (83) to (85) below: ##EQU34## In the above equations, ##EQU35## means that the sum is calculated for terms as many as possible until the convergence of the sum attains a sufficient accuracy. The terms added by this sum correspond to the interatomic potential or the differential coefficient of the interatomic potential. As the interatomic distance increases, the value of each term generally becomes small. Therefore, ##EQU36## means that interactions are taken into consideration for sufficiently far atoms where the interatomic interactions become negligible. For the Coulomb interaction, the Ewald method may be used for the lattice sum of equations (83) and (84) to increase the calculation efficiency. The Ewald method can be also applied to calculation of lattice sum of interactions whose function is given by r^(-n). When e⁰ (h0) and f⁰ (h0).sub.α are used, a potential energy E⁰ _(cell) per unit cell U⁰ and an α component F⁰ (h0).sub.α of the force acting on an atom (h0) can be calculated as follows: ##EQU37##

    F.sup.0 (h0).sub.α =A.sup.0 (h0)f.sup.0 (h0).sub.α(87)

The processing operation of the periodic system S¹ calculation unit 4-6 shown in FIG. 14 will be described next. The periodic system S¹ calculation unit 4-6 calculates a potential energy E¹ _(cell) per unit cell U¹ of the periodic system S¹ and an a component F¹ (k0).sub.α of the force acting on the atom (k0) as follows: ##EQU38##

    F.sup.12 (k0).sub.α =A.sup.1 (k0)f.sup.1 (k0).sub.α(89)

where e¹ (k0) and f¹ (k0).sub.α are given by the following equations, respectively: ##EQU39## e⁰ (k0) of equation (90) and f⁰ (k0).sub.α of equation (91) are physical quantities of the reference system S⁰. Equations (93) and (94) hold on the basis of the correspondence list [S⁰ S¹ ] and the periodicity of the reference system S⁰ :

    e.sup.0 (k0)=e.sup.0 (hζ)=e.sup.0 (h0)                (93)

    f.sup.0 (k0).sub.α =f.sup.0 (hζ).sub.α =f.sup.0 (h0).sub.α                                          (94)

These physical quantities are equivalent to e⁰ (h0) and f⁰ (h0).sub.α calculated by the reference system S⁰ calculation unit 4-5 on the basis of equations (83) and (84). In the above equations, ##EQU40## means that sum is calculated for all atoms (k'ξ') contained in a region R_(cut) centered on an atom (kξ). This region is given as a sphere having the radius R_(cut). The region R_(cut) may be designated by the user or appropriately set by the materials design system of the present invention.

As will be described later, when the region R_(cut) is set considering the balance between the computational time and computational accuracy required by the user, efficient MD calculation for the periodic system S¹ can be realized. During the MD calculation, generally, the total energy (kinetic energy+potential energy) of the system is monitored to confirm the energy conservation law. In the calculation method of the present invention, the kinetic energy can be calculated as follows: ##EQU41## Accordingly, the total energy per unit cell U¹ of the periodic system S¹ can be obtained as K¹ _(cell) +E¹ _(cell).

The processing operation of the periodic system MD calculation unit 4-8 shown in FIG. 14 will be described next. In this embodiment, the Verlet method will be exemplified as the algorithm (difference method of Newton's equation) in MD calculation, and microcanonical ensemble (the number of particles, volume, and energy are constant) will be exemplified as an ensemble. However, other methods can also be applied. To perform MD calculation for the periodic system S¹, the periodic system MD calculation unit 4-8 calculates the position and velocity of each of the N¹ atoms (k0) contained in the fundamental cell (ξ=0) at a time t+Δt using equations (96) and (97): ##EQU42## where F¹ (k0|t).sub.α is the α component of the force acting on the atom (k0) at a time t, which is calculated by the periodic system S¹ calculation unit 4-6 in accordance with equation (89), and Δt is the time interval of MD calculation. The value Δt may be arbitrarily designated by the user or set on the side of the materials design system of the present invention.

The periodic system MD calculation unit 4-8 repeatedly calculates the dynamic behavior of the structure of the periodic system S¹ until a time tend using equations (96) and (97). The end time tend is arbitrarily set by the user.

The processing operation in the block diagram (FIG. 14) showing the arrangement of the materials design system for executing MD calculation for the periodic system S¹ of the present invention has been described in units of blocks. Next, procedures of MD calculation for the periodic system S¹ will be described in detail with reference to flow charts.

FIG. 20 shows the procedures of MD calculation for the periodic system S¹.

First, various data (temperature T, time interval Δt, end time t_(end), region R_(cut), and the like) for MD calculation are input (step 5-1). These values may be input after step 5-2 or 5-3 except values such as the temperature T associated with processing in the following step.

Physical quantities of the reference system S⁰ are input to the reference system S⁰ processing unit 4-1 (step 5-2). The physical quantities are the lattice constant of the reference system S⁰, the number N⁰ of atoms per unit cell U⁰, the atomic coordinates r⁰ (h0).sub.α of the N⁰ atoms, the potential parameters A⁰ (h0), and the masses m⁰ (h0).

Physical quantities of the periodic system S¹ are input to the periodic system S¹ processing unit 4-2 (step 5-3). These physical quantities are the lattice constants of the periodic system S¹, the number N¹ of atoms per unit cell U¹, the atomic coordinates r¹ (k0).sub.α of the N¹ atoms, the velocities v¹ (k0).sub.α, the potential parameters A¹ (k0), and the masses m¹ (k0).

The periodic system S¹ processing unit 4-2 also prepares the correspondence list [S⁰ S¹ ] (step 5-4). As has already been described in the description of each block, for the physical quantities input to the periodic system S¹ processing unit 4-2, the values (initial values for coordinates and velocities) may be automatically set by the materials design system of the present invention on the basis of the physical quantities of the reference system S⁰ and the temperature T.

The reference system S⁰ calculation unit 4-5 calculates f⁰ (h0).sub.α (step 5-5).

The above-described processing corresponds to initial setting for MD calculation. The following steps correspond to the processing operation of repetitive calculation in the MD calculation. Each step of repetitive calculation corresponds to the time t of MD calculation. For example, the initial time is set as t=0.

First, the periodic system S¹ calculation unit 4-6 calculates F¹ (k0).sub.α (step 5-6). Next, the periodic system MD calculation unit 4-8 calculates r¹ (k0).sub.α and v¹ (k0).sub.α using F¹ (k0).sub.α (step 5-7). Accordingly, the periodic system S¹ processing unit 4-2 updates the values r¹ (k0).sub.α and v¹ (k0).sub.α (step 5-8). Processing from step 5-6 to step 5-8 is repeatedly executed until it is determined in step 5-9 that the time t reaches the time t_(end). Finally, at the time t_(end), the structure of the periodic system S¹ is output or displayed (step 5-10).

In the MD calculation of the present invention, the values r¹ (k0).sub.α and v¹ (k0).sub.α and the physical quantities calculated using these values can be output or displayed at an arbitrary time during repetitive calculation. In step 5-6, the potential energy E¹ _(cell) and the kinetic energy K¹ _(cell) can be calculated/output at an arbitrary time of user's choice. However, when the potential energy is to be calculated, e⁰ (h0) must be calculated in step 5-5 in advance.

A supplementary explanation about the flow chart (FIG. 20) of MD calculation for the periodic system S¹ will be made.

In FIG. 20, processing from step 5-6 to step 5-8 is simply represented as the processing operations of the periodic system S¹ calculation unit 4-6, the periodic system MD calculation unit 4-8, and the periodic system S¹ processing unit 4-2. When the Verlet algorithm is to be used, forces at the time t and a time t+Δt must be calculated to obtain the velocity at the time t+Δt in accordance with equation (97).

When the Verlet algorithm is to be used, the flow chart in FIG. 20 is partially changed as shown in FIG. 21. In FIG. 21, step 5-11 is added. In step 5-11, the periodic system S¹ calculation unit 4-6 calculates the force F¹ (k0|t(=0)).sub.α at the time t=0 (initial time is set as 0). Accordingly, processing from step 5-6 to 5-8 is replaced with the following processing in FIG. 21.

The periodic system MD calculation unit 4-8 calculates coordinates r¹ (k0|t+Δt).sub.α at the time t+Δt using equation (96) (step 5-12). Accordingly, the periodic system S¹ processing unit 4-2 updates the coordinate values r¹ (k0|t).sub.α to r¹ (k0|t+Δt).sub.α (step 5-13).

The force F¹ (k0|t+Δt).sub.α at the time t+Δt is calculated using the updated coordinate values r¹ (k0|t+Δt).sub.α (step 5-14). Using the value F¹ (k0|t+Δt).sub.α and the value F¹ (k0|t).sub.α which has already been calculated in step 5-14 of the preceding repetitive calculation cycle (step 5-11 when t=0), the periodic system MD calculation unit 4-8 calculates velocities v¹ (k0|t+Δt).sub.α at the time t+Δt using equation (97) (step 5-15). Accordingly, the periodic system S¹ processing unit 4-2 updates the velocity values v¹ (k0|t).sub.α to v¹ (k0|t+Δt).sub.α (step 5-16). The procedures from step 5-11 to step 5-16 change depending on the algorithm (e.g., Verlet method) to be used. According to the materials design system of the present invention, the flow chart can be appropriately changed depending on the algorithm to be employed.

Sixth Embodiment

MD calculation for Aperiodic System

In the sixth embodiment, a materials design system for executing MD calculation for an aperiodic system S² will be described.

This embodiment corresponds to the invention described in claim 2.

FIG. 15 is a block diagram showing the arrangement of the materials design system for executing MD calculation for the aperiodic system S². The materials design system comprises a reference system S⁰ processing unit 4-1, a periodic system S¹ processing unit 4-2, an aperiodic system S² processing unit 4-3, an interatomic potential processing unit 4-4, a reference system S⁰ calculation unit 4-5, a periodic system S¹ calculation unit 4-6, an aperiodic system S² calculation unit 4-7, a periodic system MD calculation unit 4-8, and an aperiodic system MD calculation unit 4-9.

In FIG. 15, the aperiodic system S² processing unit 4-3, the aperiodic system S² calculation unit 4-7, and the aperiodic system MD calculation unit 4-9 are added to the arrangement shown in FIG. 14. The reference system S⁰ processing unit 4-1, the periodic system S¹ processing unit 4-2, the interatomic potential processing unit 4-4, the reference system S⁰ calculation unit 4-5, the periodic system S¹ calculation unit 4-6, and the periodic system MD calculation unit 4-8 are the same as those shown in FIG. 14. The aperiodic system S² processing unit 4-3, the aperiodic system S² calculation unit 4-7, and the aperiodic system MD calculation unit 4-9 will be described, and thereafter, the procedures of MD calculation for the aperiodic system S² will be described with reference to a flow chart.

The processing operation of the aperiodic system S² processing unit 4-3 shown in FIG. 15 will be described first. The aperiodic system S² processing unit 4-3 holds/processes the crystal structure and the potential parameters of the aperiodic system S². In this embodiment, the structure of the aperiodic system S² is described on the basis of a deviation from a periodic system S¹.

This deviation occurs when the structure is modified in a certain region centered on a local disorder (e.g., impurity). In this embodiment, a system containing a local impurity will be exemplified as the aperiodic system S².

For the description based on such a deviation, the atoms of the periodic system S¹ are made to correspond to those of the aperiodic system S² in one-to-one correspondence using a correspondence list [S¹ S² ]. In the aperiodic system S², the periodicity of the periodic system S¹ disappears because of the local arrangement of the impurity. For this reason, the notation using atomic numbers (kξ) representing the periodicity is not appropriate, and serial numbers (i) are set for the atoms of the aperiodic system S². This processing can be performed using, e.g., a relation i=k+ξN¹. This information is stored in the correspondence list [S¹ S² ].

FIG. 17 shows an example of the correspondence among a reference system S⁰ (constituent atoms and unit cells are indicated by dashed lines), the periodic system S¹ (constituent atoms and unit cells are indicated by solid lines), and the aperiodic system S² (constituent atoms are indicated by gray bullets, and a region R_(relax) to be described later is indicated by a chain line).

The reference system S⁰ and the periodic system S¹ shown in FIG. 17 are the same as those shown in FIG. 16. The aperiodic system S² shown in FIG. 17 corresponds to the periodic system S¹ in FIG. 16 to which one substitutional impurity atom (indicated by a closed circle) is introduced. More specifically, for the atom ic arrangement of the aperiodic system S², atoms in the region R_(relax) are indicated by gray bullets (the impurity atom is indicated by the closed circle), and atoms outside the region R_(relax) are indicated by hollow bullets (open circles).

As the correspondence list [S¹ S² ] for FIG. 17, FIG. 18 shows an example in which the serial numbers (i) of 21 constituent atoms of the aperiodic system S² are registered. The 21 atoms correspond to atoms contained in the region R_(relax) represented by the chain line in FIG. 17. In the correspondence list [S¹ S² ] shown in FIG. 18, to clarify the relationship between the correspondence lists [S⁰ S¹ ] and [S¹ S² ], atoms (e.g., (k=5,ξ=0) and (k=6,ξ=0)) of the periodic system S¹, which do not correspond to the atoms of the aperiodic system S², are also listed. These atoms need not always be registered in the correspondence list [S¹ S² ].

In the example shown in FIG. 17, the region R_(relax) is smaller than the unit cell U¹ of the periodic system S¹. Even when the region R_(relax) is larger than the unit cell U¹, the same handling as described above can be performed. For the descriptive convenience, the impurity atom in the aperiodic system S² is also represented by an impurity number (σ).

The flow of the processing operation of the aperiodic system S² processing unit 4-3 will be described in associated with the initial setting stage of MD calculation. The aperiodic system S² processing unit 4-3 receives the number N^(imp) of impurity atoms, the impurity atom Nos., potential parameters A^(imp) (σ) and masses m^(imp) (σ), and the region R_(relax) where lattice relaxation is taken into consideration.

The impurity atom No. is the number of each atom of the periodic system S¹, which is substituted with an impurity atom. More specifically, when N^(imp) impurity atoms are to be generated, a set {(k₁,ξ₁), (k₂,ξ₂), . . . , (k_(Nimp),ξ_(Nimp))} is designated.

The example shown in FIG. 17 includes one impurity atom (N^(imp) =1), so only an atom {(k=2,ξ=0)} is designated. As the number N^(imp), an arbitrary number of impurity atoms can be handled. The impurity atoms to be designated may be contained in the fundamental cell (ζ=0) of the periodic system S¹ or contained in a peripheral unit cells (ξ≠0). In addition, the set {(k₁,ξ₁), (k₂,ξ₂), . . . , (k_(Nimp),ξ_(Nimp))} for designating the impurity atoms is made to correspond to a set of impurity numbers {(σ1), (σ2), . . . , (σ_(Nimp))} in accordance with the correspondence list [S¹ S² ].

The correspondence list [S¹ S² ] shown in FIG. 18 contains one impurity atom, and an impurity number (σ=1) is assigned to an atom (i=2) of the aperiodic system S² The region R_(relax) where lattice relaxation is taken into consideration will be described next. In the aperiodic system S², lattice relaxation takes place around impurity atoms. Since the relaxation amount of each atom becomes almost 0 at a position far from the impurity atoms, lattice relaxation may be calculated only in a finite region centered on the impurity atoms. As the region where lattice relaxation is taken into consideration, a sphere having a radius R_(relax) centered on the center of gravity of the impurity atoms can be assumed. This region need not always be spherical, and in this embodiment, the region where lattice relaxation is taken into consideration is generally represented by R_(relax). The region R_(relax) may be arbitrarily designated by the user or appropriately set by the materials design system of the present invention.

The aperiodic system S² processing unit 4-3 counts the number N² of atoms contained in the region R_(relax) (N² does not mean the square of N. In this specification, the square of N is represented by N×N to prevent confusion) and always N² >N^(imp). In the example shown in FIG. 17, N² =21. By way of a supplementary explanation, the number of atoms, which is necessary when the same system as the aperiodic system S² is to be handled by the supercell method, corresponds to the above-mentioned Nsc. Generally Nsc>N², because, in the supercell method, a region such as a buffer layer must be set around the region R_(relax) to reduce the interaction between disorders.

The computational amount of the calculation method of the present invention for the aperiodic system is smaller than that of the supercell method. Next, the aperiodic system S² processing unit 4-3 sets the initial values of coordinates r² (i).sub.α and velocities v² (i).sub.α from the correspondence list [S¹ S² ] and the periodicity of the periodic system S¹ on the basis of equations (98) and (99):

    r.sup.2 (i).sub.α =r.sup.1 (kξ).sub.α r.sup.1 (k0).sub.α +r.sup.1 (ξ).sub.α              (98)

    v.sup.2 (i).sub.α =v.sup.1 (kξ).sub.α v.sup.1 (k0).sub.α                                          (99)

The initial values need not always be set using equations (98) and (99) and may be appropriately set by the user. Handling of the potential parameters and masses of the aperiodic system S² will be described next. As described above, the aperiodic system S² processing unit 4-3 receives the potential parameters A^(imp) (σ) and the masses m^(imp) (σ) of the N^(imp) impurity atoms (σ). Accordingly, the aperiodic system S² processing unit 4-3 sets potential parameters A² (i) and masses m² (i) of the N² atoms (i) contained in the region R_(relax) as follows: ##EQU43##

As described above about the periodic system S¹ processing unit 4-2, each atom of the periodic system S¹ has the same potential parameter as that of the atom of the reference system S⁰, which are made to correspond to each other on the basis of the correspondence list [S⁰ S¹ ] (equation (77)). For the aperiodic system S², however, the impurity atom (σ) has a potential parameter different from that of the corresponding atom of the periodic system S¹. (equation (101)).

The materials design system can handle three types of impurities, i.e., a substitutional impurity atom, an interstitial impurity atom, and a defect. These impurity atoms are designated on the basis of the relationship in the potential parameters between the periodic system S¹ and the aperiodic system S², as shown in FIG. 19. FIG. 19 shows rules associated with handling of a mass m together with a potential parameter A. Handling of the mass m is the same as that of the potential parameter A (see the description of the reference system S⁰ processing unit 4-1, the periodic system S¹ processing unit 4-2, and the aperiodic system S² processing unit 4-3). The mass m can be regarded as one of the potential parameters A.

Of the three impurity atoms shown in FIG. 19, the interstitial impurity atom must be particularly carefully handled. An atom corresponding to the interstitial impurity atom is not originally present in the reference system S¹. To make the one-to-one correspondence based on the correspondence list [S₁ S² ], a dummy atom must be set in the periodic system S¹ (and the reference system S⁰) in advance. Since the potential parameter A¹ (and A⁰) of the dummy atom is set to be 0, the energy or force acting on each atom in the periodic system S¹ (and the reference system S⁰) does not undesirably change even when the dummy atom is taken into consideration.

The processing operation of the aperiodic system S² calculation unit 4-7 shown in FIG. 15 will be described next. The aperiodic system S² calculation unit 4-7 calculates ΔE² as for the potential energy of the aperiodic system S² and an α component F² (i).sub.α of the force acting on the atom (i) using equations (102) and (103) below:

    ΔE.sup.2 =ΔE.sup.2(a) +ΔE.sup.2(b)       (102) ##EQU44## where F.sup.1 (i).sub.α is the force in the periodic system S.sup.1. Equation (104) holds on the basis of the correspondence list [S.sup.1 S.sup.2 ] and the periodicity of the periodic system S.sup.1 :

    F.sup.1 (i).sub.α =F.sup.1 (kξ).sub.α =F.sup.1 (k0).sub.α                                          (104)

The value F¹ (i).sub.α corresponds to F¹ (k0).sub.α calculated by the periodic system S¹ calculation unit 4-6 using equation (89). For the energy, ΔE²(a) and ΔE²(b) are described as follows: ##EQU45## For the force, ΔF.sup.(a) (i).sub.α and ΔF²(b) (i).sub.α are described as follows: ##EQU46## where e¹ (i) of equation (105) and f¹ (i).sub.α of equation (107) are physical quantities of the periodic system S¹. Equations (110) and (111) hold on the basis of the correspondence list [S¹ S² ] and the periodicity of the periodic system S¹ :

    e.sup.1 (i)=e.sup.1 (kξ)=e.sup.1 (k0)                   (110)

    f.sup.1 (i).sub.α =f.sup.1 (kξ).sub.α =f.sup.1 (k0).sub.α                                          (111)

These values correspond to e¹ (k0) and f¹ (k0).sub.α calculated by the periodic system S¹ calculation unit 4-6 on the basis of equations (90) and (91), respectively. In the above equations, ##EQU47## represents the sum for the N^(imp) impurity atoms (σ). ##EQU48## means the sum for the N² atoms contained in the region R_(relax).

A supplementary explanation about ΔE² will be made below. ΔE² is a physical quantity defined by equation (112) below:

    ΔE.sup.2 =E.sup.2.sub.whole -E.sup.1.sub.whole       (112)

where E¹ _(whole) and E² _(whole) represent potential energies of the whole periodic system S¹ and the whole aperiodic system S², respectively. More specifically, ΔE² corresponds to a change in energy which is caused by the local arrangement of the impurity in the periodic system S¹. Since the aperiodic system S² has no periodical unit (unit cell), a quantity such as E⁰ _(cell) of the reference system S⁰, or E¹ _(cell) of the periodic system S¹ cannot be defined. In this embodiment, the energy of the aperiodic system S² is described using the potential energy change amount ΔE² from the periodic system S¹. In addition, the kinetic energy change amount ΔE² is calculated as follows: ##EQU49## With this calculation, the total energy of the aperiodic system S² can be obtained as a change amount ΔK² +ΔE² from the periodic system S¹. A supplementary explanation about a method of calculating equations (106) and (108) will be made. ##EQU50## of equations (106) and (108) means that the sum is calculated for terms as many as possible until the convergence of the sum attains a sufficient accuracy. A region R'_(cut) centered on the atom (i) may be assumed, and the sum ##EQU51## may be calculated only for atoms (j) contained in the region R'_(cut), and in many case, ##EQU52## is rewritten by ##EQU53## In calculation of ##EQU54## (or ##EQU55## of equations (106) and (108), interactions with the atoms (j) outside the region R_(relax) must also be calculated. Assume that the regions R_(relax) and R'_(cut) are given as spheres having radii R_(relax) and R'_(cut), respectively, and that the region R'_(relax) is given by a sphere having the radium R'_(relax) (=R_(relax) +R'_(cut)). To increase the efficiency of calculation, the correspondence list [S¹ S² ] is prepared for the N², for the N² ' atoms which are contained in the region R'_(relax), where N² '>N².

The processing operation of the aperiodic system MD calculation unit 4-9 shown in FIG. 15 will be described below. In the sixth embodiment, the Verlet method will be exemplified as the algorithm (difference method of Newton's equation) of MD calculation, and microcanonical ensemble (the number of particles, volume, and energy are constant) will be exemplified as an ensemble, as in the fifth embodiment. Other methods can also be applied. To perform MD calculation for the aperiodic system S², the aperiodic system MD calculation unit 4-9 calculates the position and velocity of each of the N² atoms (i) contained in the region R_(relax) at a time t+Δt using equations (114) and (115): ##EQU56## As in handling by the periodic system MD calculation unit 4-8, Δt is the time interval of MD calculation, and F² (i|t).sub.α is the a component of the force acting on the atom (i) at a time t, which is calculated by the aperiodic system S² calculation unit 4-7 using equation (103). When MD calculation for the aperiodic system S² is to be performed by the materials design system of the present invention, the aperiodic system MD calculation unit 4-9 calculates the change over time in the structure of the aperiodic system S² using equations (114) and (115), and simultaneously, the periodic system MD calculation unit 4-8 executes MD calculation for the periodic system S¹. The procedures of this processing will be described below with reference to flow charts.

FIG. 22 shows the procedures of MD calculation for the aperiodic system S². From step 6-1 to step 6-4, the same processing as that from step 5-1 to step 5-4 in FIG. 20 described above is performed. Next, the physical quantities of the aperiodic system S² are input to the aperiodic system S² processing unit 4-3 (step 6-5). The physical quantities are the number N^(imp) of impurity atoms contained in the aperiodic system S², the numbers of the N^(imp) impurity atoms (σ), the potential parameter A^(imp) (σ) and the mass m^(imp) (σ) of each impurity atom (σ), and the region R_(relax) where lattice relaxation is taken into consideration. Upon receiving the physical quantities, the aperiodic system S² processing unit 4-3 prepares the correspondence list [S¹ S² ] and sets the initial values of the coordinates r² (i).sub.α and velocities v² (i).sub.α and the values of the potential parameter A² (i) and the mass m² (i) of each of the N² atoms contained in the region R_(relax) of the aperiodic system S² (step 6-6). Next, the reference system S⁰ calculation unit 4-5 calculates f⁰ (h0).sub.α (step 6-7). The above processing corresponds to initial setting for MD calculation, and the following steps correspond to the processing operation of repetitive calculation according to the MD calculation. First, the periodic system S¹ calculation unit 4-6 calculates F¹ (k0).sub.α (step 6-8). The periodic system MD calculation unit 4-8 calculates r¹ (k0).sub.α and v¹ (k0).sub.α using F¹ (k0).sub.α (step 6-9). Next, the aperiodic system S² calculation unit 4-7 calculates F² (i).sub.α (step 6-10). The aperiodic system MD calculation unit 4-9 calculates r² (i).sub.α and v² (i).sub.α using F² (i).sub.α (step 6-11). The periodic system S¹ processing unit 4-2 updates the values r¹ (i).sub.α and v¹ (i).sub.α (step 6-12), and the aperiodic system S² processing unit 4-3 updates the values r² (i).sub.α and v² (i).sub.α (step 6-13). Processing from step 6-8 to step 6-13 is repeatedly executed until it is determined in step 6-14 that the time t reaches the time tend. Finally, at the time tend, the structure of the aperiodic system S² is output or displayed (step 6-15). In the MD calculation of the present invention, the values r² (k0).sub.α and v² (k0).sub.α and the physical quantities calculated using these values can be output or displayed at an arbitrary time during repetitive calculation. In step 6-10, the potential energy ΔE² and the kinetic energy ΔK² can be calculated/output at an arbitrary time of user's choice. However, when the potential energy is to be calculated, e⁰ (h0) is calculated in step 6-7, and e¹ (k0) is calculated in step 6-8 in advance. As described above, when MD calculation for the aperiodic system S² is to be performed by the materials design system of the present invention, MD calculation for the periodic system S¹ is also simultaneously executed. Therefore, for the periodic system S¹ as well, the values r¹ (k0).sub.α and v¹ (k0).sub.α and physical quantities calculated using these values can be output or displayed at an arbitrary time, the values E¹ _(cell) and K¹ _(cell) can be calculated/output at an arbitrary time (step 6-8), and the final structure of the periodic system S¹ at the time tend can be output or displayed (step 6-15). The flow chart shown in FIG. 22 can be changed, like the change from FIG. 20 to FIG. 21, in accordance with the algorithm to be used (e.g., Berle's method). In the materials design system of the present invention, for the aperiodic system S² as well, MD calculation according to the algorithm to be used can be appropriately executed.

Seventh Embodiment

A modification of the materials design systems of the fifth and sixth embodiments will be described below as the seventh embodiment.

Handling of interatomic interactions other than the function given by equation (79) will be described first.

The r^(-n) functions (particularly, Coulomb interaction) described by equation (79) are long-range interactions and therefore have serious problems such as an increase in computational time and a computational error caused by application of the supercell method. The materials design system of the present invention is to solve these problems.

As interatomic interactions which are not described by equation (79), there are a Born-Mayer type interaction (exponential function) representing a short-range repulsive force, Valence-Force Field model (many-body force) representing a covalent bond, and the like. These interatomic interactions are generally short-range interactions.

Since these short-range interactions have no problems such as an increase in computational time and a computational error, the energies or forces of a reference system S⁰, a periodic system S¹, and an aperiodic system S² can be calculated by a general method without any problems. When the energy and force for the short-range interaction, which are calculated by the conventional method, are added to the energy and force for a long-range interaction, which are calculated by the calculation method of the present invention, MD calculation using a general interatomic interaction can be executed.

When MD calculation for the aperiodic system S² is to be performed by the materials design system of the sixth embodiment, MD calculation for the periodic system S¹ is simultaneously performed, as described above. When MD calculation for the aperiodic system S² is to be actually performed for the purpose of materials design or the like, it is expected that the MD calculation must be executed a number to times while changing the number or types of impurity atoms in the same parent material (periodic system S¹). In such a case, the same calculation is wastefully repeated a number of times for the periodic system S¹. To solve this problem, in the materials design system of the present invention, MD calculation for the periodic system S¹ is executed only once to prepare a data base associated with the physical quantities (position, velocity, and force of each atom) of the periodic system S¹ at each time. MD calculation for the aperiodic system S² can also be efficiently executed by using the database. The database is prepared in units of simulation conditions including the type of the periodic system S¹, the temperature, and the like.

When MD calculation for the aperiodic system S² is to be performed on the basis of the first embodiment, the data base is prepared in units of simulation conditions including the type of the reference system S⁰, the temperature, and the like, thereby increasing the efficiency.

A supplementary explanation about the coordinates and velocities of the periodic system S¹ and the aperiodic system S² will be made. As the basic concept of the materials design system of the present invention, the periodic system S¹ is described by the deviation from the reference system S⁰, and the aperiodic system S² is described by the deviation from the periodic system S¹. Therefore, as for the periodic system S¹ :

    r.sup.1 (k0).sub.α =r.sup.0 (k0).sub.α +Δr.sup.1 (k0).sub.α                                          (116)

    v.sup.1 (k0).sub.α =v.sup.0 (k0).sub.α +Δv.sup.1 (k0).sub.α                                          (117).

As for the aperiodic system S² : ##EQU57## where Δr¹ (k0).sub.α and Δv¹ (k0).sub.α correspond to deviations from the reference system S⁰ caused by microscopic vibration (lattice vibration) resulting from the temperature effect, and Δr² (i).sub.α and Δv² (i).sub.α correspond to deviations from the periodic system S¹ caused by the presence of the local impurity. In the fifth and sixth embodiments, r¹ (k0).sub.α, v¹ (k0).sub.α, r² (i).sub.α, and v² (i).sub.α are directly handled. However, Δr¹ (k0).sub.α, Δv¹ (k0).sub.α, Δr² (i).sub.α, and Δv² (i).sub.α may be calculated first, and then, r¹ (k0).sub.α, v¹ (k0).sub.α, r² (i).sub.α, and v² (i).sub.α may be obtained using equations (116) to (119). In this case, as the forces of the periodic system S¹ and the aperiodic system S², instead of F¹ (k0).sub.α (equation (89)) and F² (i).sub.α (equation (103)), equations (120) and (121) below are calculated:

    ΔF.sup.1 (k0).sub.α =F.sup.1 (k0).sub.α -F.sup.0 (k0).sub.α                                          (120)

    ΔF.sup.2 (i).sub.α =F.sup.2 (i).sub.α -F.sup.1 (i).sub.α                                           (121)

The periodic system S¹ calculation unit 4-6 performs repetitive calculation while replacing r¹, v¹, and F¹ of equations (96) and (97) with Δr¹, Δv¹, ΔF¹. The aperiodic system S² calculation unit 4-7 performs repetitive calculation while replacing r², v², and F² of equations (115) and (114) with Δr², Δv², and ΔF². In equations (117) and (119), v⁰ represents the velocity of each atom of the reference system S⁰. Since the reference system S⁰ is a system at the absolute zero, v⁰ are uniquely set to be 0.

According to the materials design system of the present invention, the region R_(relax) where lattice relaxation is taken into consideration, which is set in the aperiodic system S², can be appropriately changed during simulation. With this arrangement, the region R_(relax) can be optimized, and the structure of the aperiodic system S² can be efficiently calculated. For this purpose, MD calculation combined with the method described in the second embodiment is also enabled.

In the materials design system of the present invention, even when the aperiodic system S² largely deforms as compared to the periodic system S¹, highly accurate calculation can be performed. This corresponds to a case wherein the deviation Δr² (i).sub.α represented by equation (118) becomes large. When Δr² (i).sub.α exceeds a predetermined value, the calculation accuracy of equations (106) and (108) may degrade. This problem can be avoided by setting dummy atoms in the periodic system S¹ and the reference system S⁰ such that the deviation Δr² (i).sub.α becomes small. The materials design system of the present invention can perform highly accurate calculation even when the aperiodic system S² largely deforms.

For this purpose, MD calculation combined with the method described in each of the first to third embodiments can also be performed. When the structural difference between the reference system S⁰ and the periodic system S¹ becomes large because of, e.g., a large lattice vibration caused by the temperature effect (this corresponds to a case wherein the deviation Δr¹ (k0).sub.α of equation (116) increases), the calculation accuracy of equations (90) and (91) may degrade. However, this problem can also be avoided by setting dummy atoms in the reference system S⁰, as in processing for the periodic system S¹. Therefore, the materials design system of the present invention can execute MD calculation for the periodic system S¹ even when the lattice vibration caused by the temperature effect is large.

In MD calculation for the aperiodic system S², when the number N⁰ of atoms per unit cell U⁰ of the reference system S⁰ is almost equal to the number N¹ of atoms per unit cell U¹ of the periodic system S¹, calculation for the periodic system S¹ need not always be performed. MD calculation may be executed while taking only the reference system S⁰ and the aperiodic system S² into consideration. This method corresponds to the first embodiment.

In the 5th and 6th embodiments, only MD calculation has been described. However, the materials design system of the present invention can also execute MM (Molecular Mechanics) calculation. In this case, the mass m of an atom is set to be 1, and the velocity v is set to be 0. With this setting, MM calculation can be executed following the same procedures as in the description of 5th and 6th embodiments. In addition, since the reference system S⁰ and the periodic system S¹ completely become equivalent by this processing, MM calculation for the periodic system S¹ can be omitted.

According to the materials design systems of the above-described fifth to seventh embodiments, the following effects can be obtained, unlike the prior art.

For the periodic system S¹,

Shortening of the computational time

For the aperiodic system S²

Shortening of the computational time

Realization of calculation for an infinite system containing a completely isolated disorder (equivalent to the supercell method using a unit cell having an infinite size)

Realization of calculation using a real charge distribution for a system where the charge neutrality has been lost.

These effects will be sequentially described below.

Shortening of the computational time of MD calculation for the periodic system S¹ according to the fifth embodiment will be described first.

In MD calculation, most CPU time is spent to calculate the forces. When MD calculation for the periodic system S¹ is to be performed by the materials design system of the present invention, most calculation time is spent to calculate the sum ##EQU58## of equation (91). This ##EQU59## means that, when the force acting on the atom (k0) is to be calculated, the sum is calculated for atoms (k'ξ') in the region R_(cut) (given as a sphere having the radius R_(cut)) centered on the atom (k0).

In equation (91), as the interaction with the atoms (k'ξ') outside the region R_(cut), the interaction in the reference system S⁰ is approximately used (this interaction is given by the first term f⁰ (k0).sub.α).

This means that the real interaction in the periodic system S¹ is taken into consideration for atoms in the region R_(cut), and the approximate interaction in which lattice vibration caused by the temperature effect is neglected is taken into consideration for atoms outside the region R_(cut).

In the materials design system of the present invention, the region R_(cut) is appropriately set, thereby realizing efficient MD calculation for the periodic system S¹ in the following manner. As for the atoms (k'ξ') intracting with the atom (k0), the number of the atoms (k'ξ') outside the region R_(cut) is generally larger than that of the atoms (k'ξ') inside the region Rcut.

In equation (91), the calculation result for the reference system S⁰ is used as the interaction with the atoms (k'ξ) outside the region Rcut. The amount of calculation of the force of the reference system S⁰ is small because the unit cell U⁰ is small. In addition, once calculation for the reference system S⁰ is performed at the initial setting stage of MD calculation, calculation need not be performed anymore during repetitive calculation (FIG. 20).

For this reason, since the calculation result for the reference system S⁰ is used for calculation of the force of the periodic system S¹, the time required for MD calculation for the periodic system S¹ can be largely shortened.

In the conventional method (supercell method), the calculation amount of MD calculation for the periodic system S¹ is estimated as N¹ ×N¹ ×N_(all) ×N_(step). In the method of the present invention, the calculation amount is N¹ ×N_(cut) ×N_(step) (N_(cut) is the number of atoms contained in the region R_(cut)). Therefore, the calculation speed can be increased to (N¹ ×N_(all))/N_(cut) times that of the conventional method in accordance with setting of the region R_(cut).

The influence of approximation of equation (91) on the calculation accuracy will be described. Interatomic interactions are largely influenced by atoms at close positions. Therefore, equation (91) for introducing approximation only to the interactions with atoms at remote positions is plausible.

Equation (91) does not neglect the interactions with atoms outside the region R_(cut) and only assumes that the atoms are fixed at equilibrium positions (neglect vibration caused by the temperature effect). Since the physical quantities (temperature, pressure, diffusion coefficient, and the like) which are normally calculated by MD calculation are macroquantities which are averaged for a long time, it is generally considered that the physical quantities are not influenced by approximation of equation (91).

In the materials design system of the present invention, the region R_(cut) is appropriately set in consideration of the balance between the calculation accuracy required by the user and the calculation time, efficient MD calculation for the periodic system S¹ can be realized. In actual material design, it is important to execute MD calculation a number of times for a number of types of systems while changing the condition to obtain guidelines for materials design. Particularly in such a situation, the materials design system of the present invention particularly can exhibit the effect.

The effect of MD calculation for the aperiodic system S² according to the sixth embodiment will be described next. In the present invention, when MD calculation for the aperiodic system S² is to be performed, three systems, i.e., the reference system S⁰, the periodic system S¹, and the aperiodic system S² are handled. The force F² (i).sub.α of the aperiodic system S² is calculated using equation (103).

According to equation (103), F² (i).sub.α is given by three terms, i.e., the force F¹ (i).sub.α acting on an atom of the periodic system S¹ and the physical quantities ΔF²(a) (i).sub.α and ΔF²(b) (i).sub.α of the aperiodic system S². For F¹ (i).sub.α, the efficiency of calculation is increased by equation (91), as in MD calculation for the periodic system S¹.

Since ΔF²(a) (i).sub.α calculated by equation (107) is given by the physical quantity f¹ (k0).sub.α of the periodic system S¹ and the sum for N^(imp) (<<N²) impurity atoms, the calculation amount of this term is very small. Finally, ΔF²(b) (i).sub.α is given by the sum for a difference amount [ψ'(r² |i,j).sub.α -ψ'(r¹ |i,j).sub.α ] of equation (108). Since the differential amount abruptly approaches 0 as the distance from the impurity atom increases, calculation of equation (108) rapidly converges.

Due to the above reason, the efficiency of calculation of F² (i).sub.α is increased, so that the time required for MD calculation for the aperiodic system S² can be shortened.

In the conventional method (supercell method), the calculation amount of MD calculation for the aperiodic system S² is roughly estimated as N_(SC) ×N_(SC) ×N_(all) ×N_(step). In the method of the present invention, the calculation amount is (N¹ ×N_(cut) +N² ×N'_(cut))×N_(step) (where N'_(cut) is the number of atoms contained in a region R'_(cut) described in the description of the aperiodic system S² calculation unit 4-7).

N_(SC), N¹, and N² are values on almost the same order and can be omitted. In the materials design system of the present invention, the calculation speed can be increased to about (N_(SC) ×N_(all))/(N_(cut) +N'_(cut)) times that of the supercell method in accordance with setting of the regions R_(cut) and R'_(cut).

In the first to fourth embodiments, only the reference system S⁰ and the aperiodic system S² are handled. To the contrary, in the materials design systems of the fifth to seventh embodiments, the periodic system S¹ is taken into consideration in addition to the reference system S⁰ and the aperiodic system S² Therefore, even when the periodic system has the unit cell U¹ (N¹ >>N⁰) with a large size because of lattice vibration caused by the temperature effect, efficient MD calculation for the aperiodic system S² can be realized.

The second and third effects of MD calculation for the aperiodic system S² will be described below. The "calculation for an infinite system containing a completely isolated disorder" and the "calculation using a real charge distribution for a system where the charge neutrality has been lost" are realized by describing the structure of the aperiodic system S² as a deviation from the periodic system S¹.

In the materials design system of the present invention, no periodic boundary condition is set for the aperiodic system S². For this reason, no interaction is present between disorders contained in different unit cells in the supercell method, so that each disorder can be handled as a completely isolated disorder. In the supercell method, when the disorder (impurity) has charges, artificial charges (generally, unique charges) must be assigned to each constituent atom to nullify the sum of charges per unit cell. This restriction is technically required because the energy diverges in a system whose total charges per unit cell are deviated from 0. Consequently, calculation using a real charge distribution cannot be performed by the supercell method. However, in the present invention, since the periodic boundary condition is not used for the aperiodic system S², calculation using the real charge distribution can be performed even for the aperiodic system S² where the charge neutrality has been lost.

According to the materials design system of the present invention, high-speed MD calculation for the periodic system S¹ and accurate and high-speed MD calculation for the aperiodic system S² can be realized.

As has been described above, according to the present invention, when the structural stability or dynamic behavior of a periodic system or an aperiodic system is to be analyzed in materials design, high-speed and accurate calculation (simulation) can be performed.

The present invention is not limited to the above-described embodiments. The respective units described in the embodiments can be realized not only by hardware but also by software. The method described in each embodiment can be stored, as a program that a computer can execute, in a recording medium such as a magnetic disk (a floppy disk, a hard disk, or the like), an optical disk (a CD-ROM, a DVD, or the like), or a semiconductor memory and distributed. In addition, various changes and modifications can be made within the spirit and scope of the present invention.

Additional advantages and modifications will readily occurs to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

I claim:
 1. A materials design system which analyzes a structural stability or dynamic behavior of an infinite system (aperiodic system S²) having a completely isolated local disorder at an atomic or molecular level, thereby assisting materials design, comprising:a reference system S⁰ processing unit for processing a physical quantity of an infinite system (reference system S⁰) having no disorder, thereby defining the reference system S⁰ ; an aperiodic system S² processing unit for processing a physical quantity of said aperiodic system S² thereby defining the aperiodic system S², and a list for making constituent atoms of said reference system S⁰ to correspond with those of said aperiodic system S² in one-to-one correspondence to describe a structure of said aperiodic system S² on the basis of a deviation from said reference system S⁰ ; a reference system S⁰ calculation unit for calculating an energy of said reference system S⁰ and a force acting on each atom or each molecule of said reference system S⁰ ; an aperiodic system S² calculation unit for calculating an energy of said aperiodic system S² and a force acting on each constituent atom of said aperiodic system S² using the energy of said reference system S⁰ and the force acting on each constituent atom of said reference system S⁰, which are calculated by said reference system S⁰ calculation unit; and an aperiodic system MD calculation unit for repeatedly executing an MD calculation of the dynamic behavior of said aperiodic system S² using a calculation result from said aperiodic system S² calculation unit until a predetermined condition is met, wherein said aperiodic system MD calculation unit transfers the result of the MD calculation to said reference system S⁰ processing unit and said aperiodic system S² processing unit each time the MD calculation is executed, so that the physical quantity of said reference system S⁰ and aperiodic system S² are updated.
 2. A system according to claim 1, further comprising, when an analysis (MD calculation) of the dynamic behavior of said aperiodic system S² is to be performed at a finite temperature (temperature T>0 K):means for describing the structure of said aperiodic system S² as a deviation from said infinite system (periodic system S¹) having no local disorder and for describing a periodic system S¹ on a basis of a deviation from said infinite system (reference system S⁰) which contains neither a local disorder nor considers vibration of atoms which is caused by a temperature effect, said means for describing the structure of said aperiodic system S² comprising:a periodic system S¹ processing unit for processing a physical quantity of said periodic system S¹ thereby defining the periodic system S¹, and for processing a list for making constituent atoms of said reference system S⁰ (the system in which lattice vibration is not taken into account) to correspond to those of said periodic system S¹ in one-to-one correspondence to describe a structure of said periodic system S¹ on the basis of a deviation from said reference system S⁰, a periodic system S¹ calculation unit for calculating an energy of said periodic system S¹ and a force acting on each constituent atom of said periodic system S¹ using the energy of said reference system S⁰ and the force acting on each constituent atom of said reference system S⁰, which are calculated by said reference system S⁰ calculation unit, and a periodic system MD calculation unit for repeatedly executing MD calculation using a calculation result from said aperiodic system S² calculation unit until a predetermined condition is met, thereby analyzing the dynamic behavior of said aperiodic system S² as a deviation from said periodic system S¹ ; and wherein said aperiodic system S² calculation unit calculates the energy of said aperiodic system S² and a force acting on each constituent atom of said aperiodic system S² by using the energy of said periodic system S¹ and the force acting on each constituent atom of said periodic system S¹, which are calculated by said periodic system S¹ calculation unit, wherein said aperiodic system MD calculation unit repeatedly executes the MD calculation of the structural stability of said aperiodic system S² by using a calculation result from said aperiodic system S² calculation unit until a predetermined condition is met, and wherein each time said periodic system MD calculation unit and said aperiodic system MD calculation unit execute MD calculations, they transfer results of the MD calculations to said periodic system S¹ and said aperiodic system S², thereby updating the physical quantities of said periodic system S¹ and said aperiodic system S², respectively.
 3. A system according to claim 1, wherein said aperiodic system S² processing unit comprises means for changing, during simulation, a region where lattice relaxation of said aperiodic system S² is taken into consideration.
 4. A system according to claim 1, wherein said reference system S⁰ processing unit comprises means for setting a dummy atom or a dummy molecule in said reference system S⁰.
 5. The materials design system according to claim 1, further comprising an MM calculation unit for repeatedly executing MM calculations of the structural stability of said aperiodic reference system S² by using a calculation from said aperiodic system S² until a predetermined condition is met.
 6. A materials design system which analyzes a dynamic behavior of atoms or molecules in infinite system (periodic system S¹) which has no local disorder but has vibration of atoms which is caused by a temperature effect, thereby assisting materials design, comprising:a reference system S⁰ processing unit for processing a physical quantity of an infinite system (reference system S⁰) which neither contains a local disorder nor considers vibration of atoms which is caused by the temperature effect; a periodic system S¹ processing unit for processing a physical quantity of said periodic system S¹ and a list for making constituent atoms of said reference system S⁰ to correspond with those of said periodic system S¹ in one-to-one correspondence to describe a structure of said periodic system S¹ on the basis of a deviation from said reference system S⁰ ; a reference system S⁰ calculation unit for calculating an energy of said reference system S⁰ and a force acting on each atom or each molecule of said reference system S⁰ ; a periodic system S¹ calculation unit for calculating an energy of said periodic system S¹ and a force acting on each constituent atom of said periodic system S¹, by partly substituting said energy and force of said periodic system S¹ for said energy and force of said reference system S⁰, which are calculated by said reference system S⁰ calculation unit; and a periodic system S¹ MD calculation unit for repeatedly executing MD calculations of the dynamic behavior of said periodic system S¹ until a predetermined condition is met, wherein the MD calculation unit transfers the result of the MD calculations to said periodic system S¹ processing unit each time the MD calculation is executed so that the physical quantities of said periodic system S¹ are updated.
 7. A system according to claim 6, wherein said reference system S⁰ processing unit comprises means for setting a dummy atom or a dummy molecule in said reference system S⁰.
 8. A storage medium which stores a computer program for causing a computer system to analyze a structural stability or dynamic behavior of an infinite crystal system (aperiodic system S²) having a completely isolated local disorder at an atomic or molecular level to assist materials design, comprising:a reference system S⁰ processing procedure code of processing a physical quantity of an infinite system (reference system S⁰) having no disorder, thereby defining the reference system S⁰ ; an aperiodic system S² processing procedure code of processing a physical quantity of said aperiodic system S² thereby defining the aperiodic system S² and for processing a list of making constituent atoms of said reference system S⁰ to correspond with those of said aperiodic system S² in one-to-one correspondence to describe a structure of said aperiodic system S² on the basis of a deviation from said reference system S⁰ ; a reference system S⁰ calculation procedure code of calculating an energy of said reference system S⁰ and a force acting on each atom or each molecule of said reference system S⁰ ; an aperiodic system S² calculation procedure code of calculating an energy of said aperiodic system S² and a force acting on each constituent atom of said aperiodic system S² using the energy of said reference system S⁰ and the force acting on each constituent atom of said reference system S⁰, which are calculated by said reference system S⁰ calculation procedure code; and an aperiodic system MD calculation procedure code for repeatedly executing an MD calculation of the dynamic behavior of said aperiodic system S² using a calculation result from said aperiodic system S² calculation procedure code until a predetermined condition is met, wherein said aperiodic system MD calculation procedure code transfers the result of the MD calculation to said reference system S⁰ procedure code and said aperiodic system S² procedure code each time the MD calculation is executed, so that the physical quantity of said reference system S⁰ and aperiodic system S² are updated.
 9. A medium according to claim 8, further comprising, when an analysis (MD calculation) of the dynamic behavior of said aperiodic system S² is to be performed at a finite temperature (temperature T>0 K):a procedure code of describing the structure of said aperiodic system S² as a deviation from said infinite system (periodic system S¹) having no local disorder and for describing a periodic system S¹ on a basis of a deviation from said infinite system (reference system S⁰) which contains neither a local disorder nor considers vibration of atoms which is caused by the temperature effect, said procedure code of describing the structure of said aperiodic system S² comprising, a periodic system S¹ processing procedure code of processing a physical quantity of said periodic system S¹ thereby defining the periodic system S¹, and for processing a list for making constituent atoms of a reference system S⁰ (the system in which lattice vibration is not taken into account) to correspond to those of said periodic system S¹ in one-to-one correspondence to describe a structure of said periodic system S¹ on the basis of a deviation from said reference system S⁰, a periodic system S¹ calculation procedure code of calculating an energy of said periodic system S¹ and a force acting on each constituent atom of said periodic system S¹ using the energy of said reference system S⁰ and the force acting on each constituent atom of said reference system S⁰, which are calculated by said reference system S⁰ calculation procedure code, and a periodic system MD calculation procedure code for repeatedly executing MD calculation using a calculation result from said periodic system S² calculation procedure code until a predetermined condition is met, thereby analyzing the dynamic behavior of said aperiodic system S² as a deviation from said periodic system S¹ ; and wherein said aperiodic system S² calculation procedure code calculates the energy of said aperiodic system S² and a force acting on each constituent atom of said aperiodic system S² by using the energy of said periodic system S¹ and the force acting on each constituent atom of said periodic system S¹, which are calculated by said periodic system S¹ calculation procedure code, wherein said aperiodic system MD calculation procedure code repeatedly executes the MD calculation of the structural stability of said aperiodic system S² by using a calculation result from said aperiodic system S² calculation procedure code until a predetermined condition is met, and wherein each time said periodic system MD calculation procedure code and said aperiodic system MD calculation procedure code execute MD calculations, they transfer results of the MD calculations to said periodic system S¹ and said aperiodic system S², thereby updating the physical quantities of said periodic system S¹ and said aperiodic system S², respectively.
 10. A medium according to claim 8, wherein said aperiodic system S² processing procedure code comprises a procedure code of changing, during simulation, a region where lattice relaxation of said aperiodic system S² is taken into consideration.
 11. A medium according to claim 8, wherein said reference system S⁰ processing procedure code comprises a procedure code of setting a dummy atom or a dummy molecule in said reference system S⁰.
 12. The storage medium according to claim 8, further comprising an MM calculation procedure code for repeatedly executing MM calculations of the structural stability of said aperiodic reference system S² by using a calculation from said aperiodic system S² until a predetermined condition is met.
 13. A storage medium which stores a computer program for causing a computer system to analyze a dynamic behavior of atoms or molecules in an infinite system (periodic system S¹) which has no local disorder but has vibration of atoms which is caused by a temperature effect, thereby assisting materials design, comprising:a reference system S⁰ processing procedure code of processing a physical quantity of an infinite system (reference system S⁰) which neither contains a local disorder nor considers the vibration of atoms which is caused by the temperature effect; a periodic system S¹ processing procedure code of processing a physical quantity of said periodic system S¹ and a list for making constituent atoms of said reference system S⁰ to correspond with those of said periodic system S¹ in one-to-one correspondence to describe a structure of said periodic system S¹ on the basis of a deviation from said reference system S⁰ ; a reference system S⁰ calculation procedure code of calculating an energy of said reference system S⁰ and a force acting on each atom or each molecule of said reference system S⁰ ; and a periodic system calculation procedure code of calculating an energy of said periodic system S¹ and a force acting on each constituent atom of said periodic system S¹ by partly substituting said energy and force of said periodic system S¹ for said energy and force of said reference system S⁰, which are calculated by said reference system S⁰ calculation procedure code; and a periodic system S¹ MD calculation procedure code for repeated executing MD calculations of the dynamic behavior of said periodic system S¹ until a predetermined conditions is met, wherein the MD calculation procedure code transfers the result of the MD calculations to said periodic system S¹ calculation procedure code each time the MD calculation is executed so that the physical quantities of said periodic system S¹ are updated.
 14. A medium according to claim 13, wherein said reference system S⁰ processing procedure code comprises a procedure code of setting a dummy atom or a dummy molecule in said reference system S⁰. 